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HAND-BOOK 

FOR YOUNG AND OLD 

OPTICIANS. 


A CONCISE AND COMPREHENSIVE TREATISE ON THE THEORY OF THE 
OPTICAL TRADE AND OF ITS MECHANICAL MANIPULATIONS. 


An Indispensable Companion To All Progressive Co-Laborers 

OF THE 

OPTICAL TRADE, 

CONTAINING MANY POINTS HERETOFORE UNEXPLORED AND UNEXPLAINED. 


BY 

\\\ \sKv~'T\ 

W. BOHNE, 

OPTICIAN. 


WITH ILLUSTRATIONS. 


PUBLISHED BY THE AUTHOR, 

CWith A. B. GRISWOLD & CO.) 

No. 119 CANAL STREET, NEW ORLEANS, LA. 

1888 . 







F?t 7 A'/ 

• 647 


J 





Entered according to Act of Congress, in the year 1887, by 
THE AUTHOR, 

In the office of the Librarian of Congress, at Washington, D. C. 




- ♦>« 


1 \. V 


v'+-> hi,.,- O' - 0 *- . 










‘PRESS” PRINT. 50 BIENVILLE ST.. N. a 












TO 

H. GINDEB, Esq., 

MEMBER OF THE MERCANTILE FIRM OF 

A. B. Griswold & Co., 

OF NEW ORLEANS, 

As a feeble expression of my great esteem for his sterling 
integrity, a grateful acknowledgment of his encouragement 
in furthering my present project, and as a token of my sin¬ 
cere friendship, this little work is 


Bespectfully Dedicated 



By the Author. 




ABBREVIATIONS. 


D = 

diopter. 

ax = 

axis. 

C or cyl. = 

cylindrical. 

cc = 

concave. 

cm = 

centimeter. 

cx = 

convex. 

m = 

meter. 

mm = 

millimeter. 

S or sph. = 

spherical. 

— = 

concave. 

+ - 

convex. 

3 — 

combined with. 

o = 

degree. 

/ _ 

foot. 

n _ 

inch. 

/// ___ 

line. 


ERRATA. 


Page 19, line 20, for — 40 D read — 40" or 1 D. 

Page 37, figure a b should be replaced by tlie following, 
which shows the incorrect setting of one of the prisms. 



Page 104, line 16, for Spectacle-glasses read Spectacle-lenses. 









CONTENTS. 


PAGE. 

CHAPTER L—Introductory Remarks, Diopter or Diop¬ 
tric, Inch and Metric Systems . 13 

“ II.—Different Qualities of Lenses. 21 

“ TT1 _—Pebbles—Their Merits and Defects. 28 

“ IV.—Definition of Spherical, Prismatical and 

Cylindrical Lenses. 35 

“ V.—Optic Line and Centre, and How to De¬ 
centre Lenses. 41 

“ YL—Fitting of Spectacle-Glasses, Their Filing 

and Drilling. 47 

“ YTL—Measuring of Compound Lenses and 

Their Correct Setting. 54 

“ TILL—Selection of Spectacles; Pupil Distance, 

Xose-Bridge, Reading and Street-Glasses 62 
“ IX.—Double-Focus Single and Split-Glasses, 

Relative to Optical Line and Centre ... 68 

“ X.—Colored or Tinted Glasses. 72 

“ XL—The Proportion of Caloric Rays in the 

Different Kinds of Light. 78 

“ XII.—Second Sight, its Cause and Final Course. 83 

“ XHT.—History of the Inyention and Introduc¬ 
tion of Spectacles. 85 

“ XIY.—Miscellaneous Appendices: 

A. —How To Relieve an Injured Eye .. 94 

B. —How Far Can Me See?. 97 

C. —XVhy Do Me Shed Tears Mhen Me 

Veep?. 100 

D. —Different Expressions of Faces, 

Based upon the Position of the 

Eyes. 102 

E. —Refraction and Dispersion of Light. 103 

F. —Conclusion, Containing Some Prac¬ 

tical and Useful Remarks. 105 

















PREFACE. 


When I first devoted myself to the pursuit of an opti¬ 
cian by selling spectacles and fitting glasses-, I found my¬ 
self confronted by many difficulties. I looked around 
for an instructor in the shape of a practical “Hand-Book,” 
such as I present here to young opticians. I vainly 
searched in book-stores and in catalogues; I asked ocu¬ 
lists and other authorities for a book written especially for 
opticians. But all I found were not such as I was so much 
in need of. I knew the theoretical part of our trade 
from former studies, as well, I believe, as it was known 
at that time; but as to the practical part of the trade I 
had embraced, I never could find any guide up to the 
present day. I may say, therefore, that my book fills a 
want keenly felt by all opticians. 

There is still fresh in my memory the great mortifica¬ 
tion I once suffered from my ignorance of the optic centre 
in spectacle lenses. An elderly lady wanted me to ex¬ 
change the lenses in her spectacles for stronger ones, and 
we soon found the right number. The lady was delighted 
with them, and took a seat while I was busily engaged in 
cutting and grinding them. At that time I did not use 
any pattern or marker.. I commenced to chip the lenses at 
random, as near the shape of the frame as possible. 
Unluckily one lens broke out too close to the centre; but 
as the remainder of the lens was large enough to fill the 
frame, I finished it off, and handed the spectacles to the 
lady, expecting the same approval of my work as she 
had before bestowed on the good selection of the lenses. 
She put the spectacles on, and looked at some print, but 



— 8 — 


soon took them off, wiped her eyes and tried again. She 
moved the paper and turned her head, while I was wait¬ 
ing impatiently for this approval. But imagine my sur¬ 
prise when she asked me if they were the same glasses 
she had tried before, and if I had not made a mistake. 
To satisfy myself, I remeasured them, and told her that 
they were all right. She insisted on having her old 
glasses replaced, and left my store with the remark that I 
could not fit her eyes. I would have paid, at that moment, 
one hundred dollars to anyone who had cleared up this 
mystery to me, and by showing me where I had failed, 
had saved me from another similar humiliation. From 
that time I took a memorandum of everything I thought 
of any value ’to our trade, and I have made use in this 
book of items written twenty-five years ago. 

My object is to instruct the rising generation of our 
trade, and elevate them to the position of the great pro¬ 
gress our business has made within the last quarter cen¬ 
tury. I regard my readers as my apprentices at the work¬ 
bench and behind the counter . If this work should acci¬ 
dentally fall into the hands of fellow-craftsmen of supe¬ 
rior skill and experience, I am certain that even these 
fortunate men will find something in it which will fully 
repay them for their outlay. I am well a^are that it is 
not as complete as it ought to be, because every chapter 
is composed and written as something new. There is 
nothing previously published about these subjects, as far 
as I know; and my little book may be the pioneer to 
open the road for other more able writers. All other 
trades have their literature; every other art has a hand¬ 
book of the secrets peculiar to its business; but the opti¬ 
cal trade, as regards the mechanical part of it, has none 
whatever. Books written for our instruction treat only 
of the theoretical part, but none tell us how to set a lens, 
how to find the optical centre, etc. Their writers were 


— 9 


not practical workmen; they had to limit their instruc¬ 
tions to the selection of glasses for certain special cases. 
This theoretical part I have purposely declined to copy 
from other authors, and to enlarge and embellish my book 
with strange feathers. That which I offer is the result of 
a life-long experience, and of numerous investigations; 
and I hope it will be received by the trade with favor and 
forbearance. Workmen who find any error, or who know 
better methods of accomplishing anything in an improved 
style, are cordially invited to communicate it to the 
author, who will not fail to take notice of it, and ac¬ 
knowledge his obligation in a future edition. As I desire 
to make this little work as.useful as possible, I sincerely 
request every one, who can offer any suggestion for its 
improvement, to give me the benefit of his experience. 
It is time for the optical trade to arouse from the lethargy 
with which w T e have carried on our business. Let us look 
at the immense progress the oculists have lately made ! 
If we do not keep step with them, and are not able to till 
their orders, who is to blame? 

Let us remove that curse of all progress—the keeping 
of our secrets and little tricks to ourselves. Let every 
workman withdraw the restriction upon fellow-laborers 
from entering his shop, in order to prevent them from 
profiting by nis skill. It is not knacks and tricks that 
constitute the value of a workman; but skill, judgment 
and quick perception must be the only distinction between 
the conscientious and careless, the good and bad workmen. 
This is the proper way to elevate our trade to a command¬ 
ing position, based upon skill and merits, that we may no 
longer be confounded with street-fakirs and mere spec¬ 
tacle-vendors. 

The contents of this book furnish to any young man a 
solid foundation of what he ought to know, and enable 
him to master all difficulties he may encounter in the pur- 


2 


—10 


suit of his occupation. It would be folly for anyone not 
to try and improve on w T hat he finds here; the brightest 
medical student will be but a poor practitioner even after 
ten years, unless by constant study he keeps at the head 
of his profession. This is the age of electricity, and 
electricity travels fast. There is no telling what demand 
will be laid on the ability Of the optician in the immediate 
future. But as long as he understands thoroughly the 
fundamental laws of his trade, there is nothing that can 
puzzle him more than a moment to master it completely. 
A mechanic who skillfully handles his hammer, his file, 
drill and turning-lathe, is not embarrassed by any order 
different from his routine-work. In a short time he feels 
himself at home, performing his work as easily as if he 
had done nothing else all his life-time.. This is the great 
advantage of a competent workman. Like the tools that 
he handles he says nothing, but he can be relied upon for 
successful work. He is totally different from those ar¬ 
rogant members of the trade who pretend to be brimful 
of secrets, but who are poor in execution. 

One of the fundamental points of our trade is a clear 
conception of the optical centre of a lens, as explained in' 
Chapter V; and no optician can lay claim to his title if he 
is not fully familiar with it. If this book should fall into 
the hands of the foremen of manufactories, I hope it 
may enlighten them for the benefit of the spectacle- 
wearing public; because, on an average, ten per cent. of 
the lenses; even of the better classes of goods, are mis¬ 
fits. This greatly annoys all conscientious opticians, 
and inflicts serious injury upon those who have the mis¬ 
fortune to purchase.such lenses, and to wear them with¬ 
out correction. 

Another important and more difficult point is explained 
in Chapter VII, which treats of “Compound lenses, their 
measurement and correct setting.” Since the oculists 


—11 — 


have succeeded in correcting those irregularities of the 
cornea, called astigmatism, by means of cyl. lenses, and 
have become able to determine its degree by paralyzing 
the muscle of accommodation with atropia, we have been 
compelled to combine cyl. lenses with cx and cc lenses. 
This was done, at first, simply by gluing two lenses to¬ 
gether with Canadian balsam. But opticians soon learned 
to grind both corrections on one lens, and are now able to 
add even a prism to these spherical and cylindrical com¬ 
binations. The oculists were indeed well sustained by 
the skill of our trade; and I think the honor of this 
achievement should be equally divided between the ocu¬ 
lists and opticians; as it is customary, when we admire 
the solid structure of a handsome edifice, to give credit 
to both the architect and the builder.—This chapter ex¬ 
plains in a manner easily understood, all the difficulties 
connected w T ith this most delicate correction in the shape 
of spectacles; a subject which is yet, for many opticians, 
the stumbling-block to their efficiency and ability. This 
should not be so; for often our trade is blamed in solido 
for the incapacity of individuals. 

Chapter III has for its object the much abused 4 ‘Peb¬ 
bles,” and is something altogether new.. It differs from 
everything heretofore published; and I hope that my ex¬ 
periment will be repeated by opticians and scientists, in 
order to settle finally the vexatious question: Shall 
pebbles be used or not? This matter should be decided 
under any circumstances, and the sooner, the better. If 
pebbles are really injurious, they must be discarded, as 
has been the case with many renowned remedies and 
medical treatments, much favored at one time, and after¬ 
wards thrown aside as useless. My theory about pebbles 
may be wrong, but the test I produce in their favor is 
very convincing, and I am anxious to hear what their op¬ 
ponents have to say, and are able to prove. 


— 12 — 


The history of the “Invention and Introduction of 
Spectacles ’ 9 is the first attempt at collecting the scanty 
materials about this important subject, and is far from 
being what its title indicates; but I hope that those of my 
readers who are in possession of any facts concerning 
this matter, will kindly communicate them to me for 
future use. 

. The articles in the Appendix do not belong strictly to 
the tenor of my book; but they contain some philosoph¬ 
ical-truths referring to the eyes in general, and may be of 
practical value to my readers, especially the article giving 
instructions for the relief of injured eyes. 

The list of Contents shows that every essential part of 
our trade has been discussed as far as spectacles are’ con¬ 
cerned. It may be urged as an objection against the 
completeness of this work, that I omitted to notice the 
modern machines now used in the larger establishments 
and manufactories. This is no oversight or ignorance on 
my part. I have not labored for the instruction of manu¬ 
facturers, but for the benefit of the many thousands 
of small dealers who buy their spectacles ready-made, 
and provide themselves for exceptional cases with frames 
and lenses, as they are offered by wholesale houses. 
These retailing opticians and jewelers are almost helpless 
in many respects without such a guide; and to enable them 
to give satisfaction to their customers,as well as to promote 
their interest in this particular branch of business, by re¬ 
moving for them insurmountable difficulties, I have pre¬ 
sented the results of my experience in a practice of 
thirty years as a jobbing optician. 

May this little work meet with a kind reception from 
the trade, and a mild review from the critics. 

New Orleans, Nov. 16 , 1887 . 


WM. BOHNE, Optician. 


CHAPTER I. 


Introductory Remarks — Diopter or Dioptric -— 
Inch and Metric Systems. 


Glass is the most transparent of all th§ solid substances 
produced by man, and is a good imitation of that valuable 
product of nature, rock-crystal or 44 pebbles,” which are 
but pure crystallized quartz. The Latin name for quartz 
is siiex, also silica and silicic acid , according -to certain 
distinctions mineralogists make. It is composed of fifty 
per cent, of oxygen with about an equal proportion of its 
base, called silicium ,which is supposed to be a metal, like 
potassium and sodium; but chemists cannot yet reduce 
it to its metallic state. Fifty years ago they extracted 
the base of clay , in the form of that extremely light 
metal, aluminium; but the metal silicium is yet waiting 
for its discoverer.* 

To manufacture glass , we must take quartz or sand 
(the latter is only powdered or crushed siiex), and melt 
it together with either potash or soda, with the addition 
of lime, borax, lead and other ingredients which facilitate 
its fusion. Quartz or sand by itself will never melt; it is 
perfectly infusible; but it acquires the property of 


* After having written the above, I ascertained that a commencement in this 
direction has been already made in electroplating with Silicium, obtained di¬ 
rectly from silica by means of hydrofluoric and hydrochloric acids. The metal 
Silicium is then invisibly suspended in the solution in which the article to be 
plated is immersed, and is set free by the action of a galvanic current. In this 
way we obtain a thin film which is nevertheless the real metallic base of silica. 

When we consider how tedious were the first experiments with Aluminium, 
and in what quantity and with what facility this metal is now produced, we may 
also expect to see Silicium introduced sooner or later into the market, as a new 
metal for ornamental or industrial purposes. 





— 14 — 


fusibility in a greater or lesser degree according to the 
quantity of the above metallic oxides with which it is 
mixed before undergoing the melting process. 

In reference to spectacle-lenses we have to deal either 
with pebbles, flint-glass or" crown-glass, whose different 
qualities" will be discussed in the next chapter. The 
lenses mostly used are ground spherical, and are either 
convex or concave. Cx lenses collect the greatest portion of 
the rays falling on their surface at one common point called 
the “ focus;” concave lenses, on the contrary, disperse or 
scatter the rays, and have only a negative focus. Cx 
lenses are always thicker in the middle than at the 
edges, and are of different form or shape, either double 
or bi-cx, plano-cx or periscopic convex.* Concave lenses 
are thinner in the middle than at the outside, and are 
also double or bi-concave, plano-cc and periscopic-cc. 
The advantage of one form over the other is yet an open 
question. I, for my part, find that the stronger numbers 
of periscopic lenses are extremely unpleasant to the eye, 
.especially when they are used for cataracts. All that 
is claimed for their superiority may be justified in the 
weaker numbers from 1 to 4 diopters. 

Diopter , Dioptry or Dioptric f Which is right and 
should be used ? The latter word is derived from “diop- 
trique,” applied by the French to optical measurements, 
as a substitute for the term meter , which, though it de¬ 
notes measure in general, has no particular application to 
optical measurement. On the other hand, the great.ob¬ 
jection to “dioptrique” is that it denotes no measure 
whatsoever. Both in its derivation and its use it refers 
to the refraction of light, but not to any kind of meas¬ 
urement. It is not a new word, but has been assigned a new 
meaning, not in accordance with the logical rules of lan¬ 
guage. It is not a noun, but an adjective, in its form and 
meaning. The word dioptry would, from its termination, 


* For explanatory Cuts see Appendix F. 



— .15 — 


present a better claim to be used for our purpose; thus 
we translate “replique” into reply; but as most, nouns 
relating to measure end in “ter,”: from meter, thermo¬ 
meter, barometer, up to a geometer, I think it best, to 
adopt that spelling for it which I.have used throughout 
this book. 7. 

This word diopter denotes, also a geometrical instrument 
used for leveling purposes. The original word is taken from 
the Greek dioptricos (dia=through, optomai—to see):, 
meaning something which assists vision by means of the re¬ 
fraction of light. This assisting medium is always, a 
dioptric lens, or a combination of lenses, from, the tele¬ 
scope down. to. a pair of spectacles. 

The substitution of this word for meter has been 
adopted by oculists and all first-class opticians since 18 1% 
in the interest of the legitimate trade, partly to prevent 
bunglers from filling orders "of. oculists, and from taking 
advantage of the fruits of science. without troubling 
themselves to study and become: experts in the optical 
trade, and also to adopt a uniform measurement instead 
of: the old inch measure, which differs according to its 
length in different countries. . , 

Spectacle-lenses are never mathematically correct, like 
those of scientific instruments, because nobody would pay 
the price for them, and, in fact, nobody would detect a 
great difference or be much benefitted by them. Just 
see how most people put on their glasses. How. carelessly ! 
How crooked ! They have no more use for such perfect 
lenses than an Indian has for classic music. 

When the lenses are well centred, the focus will be a 
sharp point in sunlight ; otherwise it will form a circle, 
an ellipse, or only an irregular patch of light. The 
very common spectacles are always badly centred, 
and never correctly measured, and should-not-be., sold 
by any conscientious optician. If people are willing to 


—• 16 — 


injure their eyesight for the sake of a few dimes, let 
them do so; but you should rather lose the sale of an 
article than be parties to such reprehensible dealings. 
Druggists are forbidden to sell poison; wffiy does not the 
law prevent the traffic in “ eye-killers’ J ? 

The first qualification of an optician is his ability to 
measure lenses. The introduction of a new measure 
only confuses people, so long as they have not forgotten 
the old style of measuring. We experience the same 
difficulty with the metric system. A diopter has, in 
American measure, 39.37", and in Paris, 37"; only 
the inches are different, not the meter. To find the 
difference between an American and a French foot in 
lines, multiply the number of lines in one foot (144"') 
by 37, and divide the product by 39.37. The quotient 
will be 135.3"', which is the length of an American 
foot in French lines. The French foot of 144"' is there¬ 
fore 8. 7'" longer than the American.* 

This explains why imported lenses never correspond 
with our numbers, and have to be remeasured. When 
w T e order +20, we find them generally to be +22; and it is 
only by keeping in stock the half numbers, as 5J, 6£, etc., 
that we are able to fill correctly the orders of oculists. 
This trouble is overcome by the metric system , as a 
meter is independent of the special measurements of the 
different countries. 

Let us now turn our attention to the practical method 
of measuring lenses; and first, by the inch system. If 
you have in your store or workshop a suitable place to 
fasten permanently a ruler of 40" in length, horizontally, 
with a white card attached at zero, and counting from 
there in the direction of a conspicuous object, for 
instance a window or a railing, it is easy to find the 


fhe French inch is 27.07 m m. the American only 25.3 m m. 



— 17 - 

focus by moving the lens to and fro till you have a well 
defined picture of the windows etc., on the card. This 
figure is always reversed, the reason for which will be 
explained in another chapter. As soon as the figure 
shows clearest, you count on the ruler the number of 
inches, which will be also the number of the lens. 
There is no difficulty in measuring, in this way, convex 
lenses up to 30"; beyond that, it requires greater care, 
and some practice to distinguish the faint picture on the 
card, especially in cloudy weather. The surest way to 
measure weaker lenses than + 30, is to place two together 
and measure them jointly. Two lenses-of + 48, will 
give + 24, and one lens separate will be again +48, or 
half the strength of + 24. But if we have two lenses 
apparently of the same strength, which are actually + 60 
and + 72, how can we ascertain that they are of different 
strength? Our ruler will be useless to us, even if we 
lengthened it sufficiently. Here we have to fall back 
on our own eyes, and let them render judgment. Take 
for instance a folding foot-rule of 2' and open it enough 
to just introduce a pin-head between the open ends. 
You will hardly think that this little opening has much 
effect on the parallelism of the tw r o lines. But now 
place this foot-rule so that the continuation of one 
branch strikes a point several hundred feet away from 
you; then, without moving the ruler, follow with your eye 
the other line, and you will see wffiat effect this little 
opening has. You will understand by this, that you 
have to compare such weak lenses by looking at remote 
objects. If there is convenient a roof of a house one or 
more hundred feet away from you, take the two lenses, 
+ 60 and+ 72, one in each hand; hold them edgewise 
together, and look through them at arm’s length at the 
roof; move one or the other lens up or down till you see 
the lower line of the roof straight through both glasses. 


- 18 — 

Now look, without moving the lenses, at the upper line., 
and you will find that it is higher in one lens, and this 
is of course the stronger one, as it is of greater magnify¬ 
ing power. 

There is a certain method to measure, cc lenses; but 
I, for my part, have practiced it only as an experiment.. 
When the sun shines upon a ce lens, we do not see a clear 
focus, but only a shadow of a certain dimension. The 
more we move the lens toward the sun the larger will 
be the shadow; and if we continue to move it till, the 
shadow is just double the size of the lens, this distance, 
in inches, is its strength. This method however is of no 
practical value. As we are able to measure cx lenses very 
accurately, we can use them as a standard measure for ce 
lenses by neutralizing the cc lens with its corresponding 
cx lens. If you have any cc lens, of which you do not 
know the number, pass before it different numbers of 
cx lenses till you come to that lens which makes them 
both together appear plane, and the number of the 
cx lens is then the number of the concave one: -f 7 and 
—7 is 0 or plane. 

The metric system is based on the length of the meter 
which is very nearly 40 inches. The great advantage of 
this new measurement is that it enables us to make calcu¬ 
lations and combinations of lenses without the least 
trouble. The old way of calculating combinations in 
inches is more or less difficult; we have to turn the num¬ 
bers of glasses into fractions, and instead of -f 40 we say 
+ 4 V; instead of + 20 , e tc. Suppose somebody is 
wearing -f F V, but cannot see well, and we add another lens 
•fioi by which combination he sees perfectly; then we 
have to make the calculation in this way: 

t V I ^ = 67 j 1080 L + 16 inches. 

"1ST 1 4 0 J l 

Not every dealer is sufficiently familiar with his 



— 19 


arithmetic to be always sure of a correct result; and it 
will be well therefore to present an easier method by 
which all possible combinations may be made by 
simple addition or subtraction. This can only be done 
in the metric system. 

One meter or a diopter is exactly 39.37 of our inches, 
so 2 diopters are the half of it—19.68". Some cata¬ 
logues of importers and manufacturers of spectacles give 
all the fractions arising; from a careful calculation 
of diopters into inches, but they are of no practical value 
whatsoever. If we take the meter at 40 inches and make 
our calculation accordingly, we will be always near enough 
for all practical purposes. To illustrate the easy way of 
making the above calculation, look only at the annexed 
table, giving the most rational comparison of diopters with 
American inches. You find there that + 27'' are 1.50 
diopters, and that 40" is one diopter; add both together 
and you have 4- 2.50 D or 4- 16". If anybody is 
wearing these spectacles (4-16"), and you find that they 
are too strong, but are corrected by adding -—40 D, 
then you have to subtract 1 D from 2.50 D, leaving 
4- 1.50 D, or 4- 27 inches. 

To give an illustration of the difficulty in making 
correct calculations by the inch system, let us take for 
instance an achromatic lens, composed of a crown-glass 
of4-4j inches and a flint glass of—7§ inches. These 
two lenses combined give one lens of 4-10; but by the 
ordinary way of adding the two sums, w r e may get a lens 
of—10 in our calculation. To solve this problem cor¬ 
rectly we have to make use of one of the first algebra¬ 
ical rules of addition, which reads: “When the signs are 
unlike, prefix the difference of the two sums with the 
sign of the greater part.” In ordinary calculation 7| 
is the greater sum; but in optics we know that 4^-is nearly 
twice as great or strong as 7§, and in adding the two 


— 20 — 


sums we have to prefix therefore the answer by the 
sign -f, as the practical test shows it to be correct . 

By means of the metric system we have not the least 
trouble in finding the value of those two lenses combined : 

4- 4J equals 4- 9 Diopters, 

— 7§ 44 — 5 “ 

added together gives 4-4 4 4 equal to 4- 10. 

The annexed table will answer for all practical purposes. 

Diopter, 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.25 2.50 
Inches : 160 80 54 40 32 27 23 20 18 16 

Diopter : 2.75 3.00 3.25 3.50 4.00 4.50 5.00 5.50 6.00 6.50 7 

Inches: 14J 13 12 11 10 9 8 7 6J 6 5J 

Diopter: 8 9 10 11 12 13 14 16 18 20 

Inches: 5 4J 4 3J 3J 3 2f 2J 2£ 2 

Between these numbers there are only 27, 23 and 14J 
not common to our inch system, and they maybe changed 
into 26, 22 and 14; all others are represented by the old 
inch measure. I do not recommend that the orders of 
an oculist be filled in an inaccurate way in order to dis¬ 
pose of the lenses we have on hand. It requires only a 
small outlay to obtain at least the above numbers in diop¬ 
ters. It is unquestionably the best policy for an optician 
to complete his stock by degrees with all numbers of the 
metric system; but in cases of necessity, we will be near 
enough if we substitute one for another, according to the 
above table. 


CHAPTER II. 


Different Qualities of Lenses. 

Many opticians are mistaken about the hardness of 
flint-glass and crown-glass. The general opinion that the 
former is the hardest, obtains even among noted writers. 
Dr. Donders, for instance, says: “ Flint-glass and rock- 
crystal are harder than crown-glass.” I do not under¬ 
stand how this error could slip into so many medical books, 
as the simple test of scratching the one with the other 
will show at once that flint-glass is softer than crown-glass. 
Dr. Donders is not so much to be blamed for this incor¬ 
rect statement as those who reproduced his error without 
any further investigation. 1 read lately in a valuable 

geographical work of Dr. H. Berghaus, that Washington 
served his country twelve years as President. I do not 
think less of Dr. B. for making this erroneous statement, 
but I censure every writer who quotes him as an author¬ 
ity on this subject. 

We find another error in regard to Pebbles repeated in 
many books written by thoughtless compilers, because one 
copies it.from the other without examining the facts; 
and as the first writer was mistaken, all the rest labor 
equally under the’same gross misrepresentation. .1 will 
correct, in the next chapter, those errors which have, for 
years, caused an open contest between oculists and opti¬ 
cians. The oculists based their objections on books of 
high authority, and the opticians yielded to their argu¬ 
ments from sheer want of correct information. I warmly 

uro-e both to devote some of their leisure to investigate 

© « • 
this question thoroughly, and settle it definitely. 


— 22 - 


Let us first see what material is necessary for the manu¬ 
facture of glass. Glass cannot be made without quartz, 
which must be crushed and powdered to sand before it 
can be melted. To avoid this labor, the manufacturer 
prefers to take at once sand of a clear grain. The mix¬ 
ture of sand and potash or soda melted together, gives 
the so-called water-glass , which is soluble in hot water, 
and is readily affected by acids; but to prevent this, 
lime is added. These three ingredients, sand, potash or 
soda , and lime or another metallic oxide, are absolutely 
necessary for glass-making. To facilitate the melting 
process, and also to produce different kinds of glass, 
there is added, what is called flux: 

1. Baryta , which renders it more easy of fusion. 

2. Manganese, termed “glass-makers soap,” which 
neutralizes the greenish tint produced by iron. 

3. Borax, must be used with great caution, as an ex¬ 
cess causes exfoliation of the glass. 

4. Lead (in the form of red lead or litharge), re¬ 
moves impurities, and is the distinguishing ingredient in 
crystal or flint-glass. 

5. Arsenic, promotes the decomposition of other in¬ 
gredients, and tends to dissipate carbonaceous impurities 
not otherwise disposed of, but is then volatilized. 

6. Alumine and Bon are seldom present, and very 
undesirable in the finer qualities. 

.There are.four varieties of glass manufactured, besides 
the water-glass: 

I. Flint-glass, also called Crystal, Strass or Paste. 
This is a very pure and beautiful kind of glass, of great 
density and high refractive power. It consists of 


- 23 - 


42.5 

parts of silica. 

43.5 

4 4 

44 oxide of lead. 

11.7 

4 4 

44 potash. 

1.8 

4 4 

44 alumine. 

.5 

4 4 

44 chalk or lime. 


100 . 


It has the highest degree of lustre, but is soft and 
easily scratched. The specific gravity is 3.7, while that 
of crown-glass is 2.7, and of rock crystal only 2.6. Many 
opticians may have confounded density with hardness, 
and have made the same mistake as they would, if they 
maintained that dense .and compact chalk was harder than 
light and porous pumice stone, although the hardness of 
the first is 1, and of the other, 7. Density is the opposite 
of rarity, but not of softness. 

II. Window Glass, English Crown and Plate Glass. 
This is made of silica, soda and lime, and is used exten¬ 
sively for spectacles of the second quality. It is com¬ 
posed, when used for optical purposes, of: 


White sand 120 parts. 

Carbonate of potash, . 35 66 

Carbonate of soda. 20 “ 

Chalk 20 “ 

Arsenic. 1 << 


196 “ 

There are 55 parts of potash and soda in 196, which is 
equal to 28 %, against nearly 12% in flint-glass. 

III. Bohemian or Crystal-Glass, made of silica, 
potash and lime. 

IV. Bottle Glass, made of silica, soda, lime, alumine 












— 24 - 


and iron. This glass is the hardest, but the most impure, 
and has always a greenish tinge. 

You see from this list that the dearer potash is used for 
the better qualities of glass, and the cheaper soda for the 
inferior kinds. If either of them is employed too freely, 
it spoils the glass. You have perhaps made the observa¬ 
tion, after having carefully cleaned a mirror or window, 
that soon there was a scum again covering the just 
brightened surfaces. A repeated rubbing readily removed 
it only to re-appear as soon as you ceased your efforts. 
The excess of potash or soda in the glass attracts the 
moisture of the air, and baffles your exertions. Don’t 
laugh any more at people complaining that they can never 
clean their spectacles; the lenses may be manufactured 
of such defective glass. 

There is another serious evil attending the excess of 
these alkalies in glass, when they gradually oxidize by 
the action of the atmosphere, causing the appearance of 
rainbow colors. But when in the length of time the potash 
and soda are more and more absorbed from the surface, 
there is left only a thin film of oxidized silica of a milky 
appearance, such as you find on spoiled lenses, especially 
On watch-glasses, which are mostly made of glass con¬ 
taining much soda. Such lenses cannot be cleaned by 
any acids or by any amount of rubbing, but only by 
being reground and repolished in the factory, which does 
not pay expenses. 

I think it proper here to direct your attention to the 
many so-called inventions of certain “smart” and un¬ 
scrupulous impostors, introducing their wonderful dis¬ 
coveries as something of the greatest importance, i. e., 
for their own pockets, not for the public. The Only in¬ 
vention they have really made, is the high sounding name 
which they first flourish ostentatiously before the eyes of 
the amazed public. Then they wait eagerly for the rush 


— 25 — 


of deluded buyers, as the picadores in the arena wait for 
the headlong advance of the bull, enraged by the waving 
of red cloth. Such names as Perfected, Improved, Bril¬ 
liant, Arundel, Diamond, Medicated, Diamanta, etc., are 
still fresh in our memories, and the list will increase as 

long as there are dupes enough living to make such a 
humbug pay. 

It is very important to every optician to be well in¬ 
formed about the different qualities of lenses; he should 
be able to determine their various grades as readily as a 
jeweler is able to ascertain the karats of goods he is 
buying. Before we speak of the proper test, let me cor¬ 
rect a common mistake in regard to the manufacturing 
of lenses. They are not always ground from square flat 
pieces of glass, but are now mostly cast or moulded into 
the shape they have when finished. Only the better qual¬ 
ities receive, afterwards, their finishing touch by ttye 
grinder and polisher, while those lenses with somewhat 
even surfaces, are only superficially polished, and are 
then ready to be used for cheap spectacles. Lenses of the 
first quality always contain more or less lead, the larger 
its quantity (to almost half its volume), the finer its 
lustre and beautiful sparkling. This kind is known to the 
trade as extra ivhite flint-glass, and cannot be distinguished 
from pebbles by simply comparing them together by look. 
It is principally used for opera and spy-glasses and other 
optical instruments:' The best method of comparing dif¬ 
ferent lenses is to place them horizontally or flat between 
.your fingers, and by holding the ha^nd towards the light, 
you can see then in the narrow open spaces between your. 
dark fingers the different degrees of the color of these 
lenses better than by placing them on white paper. But 
most lenses sold for first quality are not the extra white, 
and cannot stand comparison with pebbles; the simple 
hand-test shows a grayish tinge when compared with them. 

3 


— 26 — 

Lenses of the second quality contain no lead, have 
a greenish tinge when examined edgewise, but take 
a high polish, are harder than flint-glass, and are decid¬ 
edly preferable for cataract lenses. 

The third quality is not always made of poorer glass, 
because many lenses from the better qualities are selected 
to be used as they were cast. We find Therefore among 
them very often white lenses, but they are never ground, 
and seldom polished. Their cheapness is due more to 
saved labor than to less costly material.—I could extend 
the list of the different qualities to fourth and fifth 
grades, when I look around among the stock in trade of 
our street-corner opticians, but I hope none of my read¬ 
ers will be caught selling such trash. It is true, the eye 
can stand a great deal of abuse, but the wearer of such 
spectacles will at last share the fate of a spendthrift: the 
one loses his fortune, the other, his more precious sight. 

A good lens, no matter if made of flint or crown-glass, 
is always well ground, polished and correctly centred, 
without flaws and streaks. “ Correctly centred ” means 
that the centre of one side is exactly opposite the centre 
of the other side, the continuation of these centres form¬ 
ing a right angle with the surface of the lens, and giving 
a well-pointed f06us in sunlight. 

v a 3 -t JL 

o o o co O O 

No. 1 is perfectly centred, No. 2 not so perfectly, No. 3 
* and 4 are each of the shape of focus No. 2 when held 
slightly oblique; the centre of each side of the lens pro¬ 
duces its own focus. Nos. 5 and 6 show only irregular 
patches of light, very often seen by cast lenses. 

To detect other imperfections we have to hold the lens at 
an angle of 35 c in good light. The reflected light will show 


— 27 — 


the smallest bubble or scratch in or upon the glass. An¬ 
other and better method is to hold the lens before the eye, 
and look through it at a window. (This test refers only 
to cx lenses.) You will see the object behind it only 
dimly, and in lengthening the distance gradually, it will 
appear dimmer, till at once you see nothing but the glary 
lens,—it is just in its focal distance. If you remove the 
lens beyond this point, the object is then clearly seen but 
reversed, because the rays have crossed in the focus; the 
upper rays are now the low T er ones, and vice versa. This 
point where you see nothing behind the lens, is the most 
proper for detecting all imperfections in it; you only 
direct it to some light colored object, or towards the sky. 

This test is also the best to detect the defective crystal¬ 
lization in pebbles. 


CHAPTER III. 


Pebbles, their Merits and Defects. 

. For more than a hundred years, after cotton began to 
be cultivated in America, its seeds were considered worth¬ 
less, and on every plantation large heaps of this contemned 
stuff accumulated in the course of time, which the planter 
would have gladly given for nothing, if anybody had 
been kind enough to cart it away. To-day, the seed yields 
more profit to him than the cotton. Pebbles met with 
the same treatment. Neither the builders had any use for 
them, nor street-pavers; only mineralogists noticed them, 
and occasionally collected some specimens as cabinet- 
pieces. A few manufacturers of glass also used them for 
making an extra quality of flint-glass, but millions of 
tons of this precious substance were left unnoticed by 
those who are now eagerly searching for it. Since 1783, 
when Abbe Rochon, the first writer on pebbles, gave an un¬ 
favorable account of them, and condemned them as useless 
for spectacle lenses, all writers on the subject are against 
them. Listen to what the Doctors say: 

44 The only practical advantage of pebbles over glass is, 
that they enable us with all honesty to gratify persons 
who do not know what they want, but simply wish to pay 
more than the usual price, or more than their friends did 
for their spectacles.” 

Another says: 

44 Rock-crystal, or Brazilian quartz, is also used, and is 
commonly known as pebbles. It has no advantage over 
glass, except in hardness; in fact, the opticians find it 


— 29 - 


difficult or impossible • to distinguish between them with-* 
out a polariscope or a tile. Many people, however, are 
not satisfied unless they have pebbles, or think they have 
them, for glass is very often sold instead.” This phy¬ 
sician forgets that jewelers are also compelled to use 
touchstone and acid to test gold. Is gold therefore less 
valuable ? - - . 

I frequently tried to find some definite information 
about pebbles; but being unable to discover any book or 
pamphlet treating of this subject, either here of in 
Tuiope, I concluded to search for myself, and ascertain if 
there was anything in it to repay the labor. Only a 
superficial glance at them revealed their extreme trans¬ 
parency, and plainly showed that few spectacle-ienses 
possessed that brilliancy which characterizes these much 
abused pebbles. T asked myself the question: Why 
shall we abandon the natural, pure glass for an artificial 
substitute; the reality, for an imitationf ‘ 

The genuine article lias two striking advantages over 
glass which cannot be denied:. its T/rilliancy and its hardr 
ness. The principal objection made against the use of 
pebbles is their double refraction; but this is seen only ih 
thick pieces, when we look over their slightly inclined 
surfaces, Objects reflected from polished planes of mas¬ 
sive pieces appear double; but this.is not the case with 
thinner plates, like spectacle lenses.* Since, therefore, 
double refraction affects vision only in thick pieces and 


•* “ The doub]e refraction of rock-crystals renders them useless for optical 
purposes, and especially for the manufacture of-spectacle-lenses, and although 
the images do not appear doubled across such lenses in consequence of their 
thinness, and the manner in which they are used, it is nevertheless true that 
double refraction exists, and that it can cause considerable trouble to vision 
by weakening the retina, and producing fatigue of the accommodation or even 
a^und of ambrybpy.»-Manucl de l’Etudiant Oculiste, par Aiixauii Cheva- 





-30 - 


not in thin ones, what reason have opponents to prejudice 
the public against their use ? Why not raise their voices 
likewise against the use of small quantities of arsenic, 
belladonna and other poisons? for it is well-known that 
large doses of them have deadly effects. On the contra¬ 
ry, they,.as well as the most cautious and conscientious 
physicians, daily prescribe small doses of these poisons, 
with successful results. 

This is the only serious objection ever made against 
pebbles, and I would think it too insignificant in compari¬ 
son with their other high qualities, which give them a 
prominent place among all their competitors for spectacle 
lenses, to waste another word in their defence, if it were 
not my object here to settle the dispute definitely, and 
furnish all the points necessary to justify my honest 
opinion about them. The main object of my investiga¬ 
tion was, therefore, to ascertain, if the eyes were sooner 
fatigued with pebbles than with glasses. I directed my 
attention especially to the general cause of our getting 
weary , and I found that it was the effect of heat relaxing 
the muscles and producing the sensation of fatigue. 
People say: “ My eyes burn,” meaning that they are 
fatigued. In fact, as soon as any part of our body is 
overheated, it feels tired. A long walk produces an in¬ 
creased flow of blood into our limbs and feet, we feel 
fatigued, and find relief in cooling them. This also ex¬ 
plains why we grow tired sooner in summer than in win¬ 
ter, though doing the same amount of work. 

The proper way, therefore, to reach a positive result, is 
to measure the amount of caloric rays transmitted through 
pebbles and glasses respectively, and to find the difference 
in favor of one or the other. This test I first made six¬ 
teen years ago; and I have repeated it before writing 
this article. I give here a brief description of the 
manner in which it is done. We can measure the different 


- SI — 


degrees of heat only by means of thermometers; and in 
order to make this test simultaneously with different 
lenses, I selected six thermometers that worked accurately 
together; then I took an axis pebble , a non-axis pebble , 
a flint-glass , a crown-glass , a light smoked and an Arun¬ 
del lens , all of + 8. I made a slender frame-work to 
hold the lenses and the thermometers; then removing the 
thermometers from their casings, I placed them, one 
each, in the focuses of the lenses. To guard against any 
inequality in this test, I took a straight piece of sheet- 
iron, and had six holes punched out, all of the size of a 
silver quarter dollar, and fastened the lenses behind each 
hole so that the optical centre of the glass w T as in the 
centre of the hole. 

I took altogether thirty-two observations, with the fol¬ 
lowing result: 

The smoked lens showed 78° on the average. 

64 crown-glass, 4 4 81 ° 4 4 4 4 

44 non-axis pebble 44 81£ ° 4 4 4 4 

44 axis-pebble 44 82 ° 4 4 4 4 

44 flint-glass 44 83 ° 4 4 4 4 

44 Arundel lens 44 84 ° 4 4 4 4 

The lesson we may draw from these observations is 
that we should dispense with flint-glass and all colored 
lenses, except smoked. Crown-glass and pebbles are then 
left as the only rivals for spectacle-lenses. 

Crown-glass is not always manufactured of sufficient 
clearness to suit optical purposes; most of it contains 
iron, which gives it a greenish color ; then, too, the smai] 
proportion of potash in it prevents the sand from being 
melted into a perfectly pellucid mass. The best formula 
for making crown-glass for spectacle lenses we find in 
the preceding chapter. This glass has an index of re- 


— 32 — 


fraction of 1.538, and an index of dispersion of 0.037, 
while the best flint-glass has an index of refraction of 
1.633, and of dispersion, 0.049. When we take into 
consideration that the greater the index of refraction of 
a lens, the more dazzling it is to the eye, and the greater 
the index of dispersion, the more annoying and fatiguing, 
we cannot hesitate in making our choice. Besides, 
crown-glass is the hardest of all glasses used for spec¬ 
tacles, and is at the same time much cheaper than flint-glass: 

In reference to pebbles, we have to take notice of their 
greater index of refraction (1.548) which accounts for 
the higher stand of the thermometer in the trial-test. The 
difference in the refraction of both is but very small, and 
is fully balanced by the lower index of dispersion, which 
is only 0.026 in pebbles.' * The difference of the thermo¬ 
meter between axis and non-axis pebbles puzzled me at 
first considerably; but I think it can be fully explained 
by the presence of the prismatic colors in axis pebbles. 
The red ray is very predominant in such lenses, and as 
red is the caloric ray “ par excellence,” it explains the 
greater heat in comparison to non-axis pebbles. I believe 
this also covers the case in regard to Arundel lenses, 
which are based altogether upon a wrong theory. When 
we resolve the light by a prism into its seven colors, and 
examine the caloric of the violet ray, we find it of much 
lower temperature than that of the red ray. But when 
we produce a violet glass (which is done by the addition 
of oxide of manganese to the other ingredients), and let 
the light pass through such a'lens, the redray receivesof 
course an additional force from the somewhat reddish lens, 
which sends a warmer light to the eye than white glass 
would do. The violet ray of the spectrum is separated 
from the other rays, and is, therefore, cooler; but a violet 
lens cannot exclude the other six colors. It cannot by the 
exclusion of the other colors give the eye the benefit of 


— 38 - 




only that cool portion of the white light which is con¬ 
tained in the violet ray. 

It now remains to decide which pebbles are preferable, 
the axis or the non-axis. I am not prepared to give a 
categorical answer to this question; but I hope that more 
able writers will investigate the subject and free it from 
the false opinions entertained upon it for so many years. 
Indeed, all the ignorance of thoughtless writers have not 
availed to rob these crystals of their hardness, nor to obs¬ 
cure in the least their brilliancy and clearness. All the 
woful insinuations about their double refraction have been 
unable to double even the finest test-line in spectacle 
lenses, or to produce that great trouble in the eyes of the 
wearer which was so earnestly predicted by them. Peb¬ 
bles are used to-day, and will be used in future, as long 
as crystals can be found; but they should be tested more 
scientifically and repeatedly, before we can give one or the 
pther the preference. Meanwhile I advise all opticians to 
introduce either of them without the least hesitation, but 
to dispose only of those pebbles which are faultless as to 
their crystallization. There are many pebbles in the 
market full of imperfections; they should not be used, 
but thrown aside. 

In rehearsing the merits of pebbles, we find that their 
hardness and clearness surpass those of any glass, and 
that their dispersing power is the lowest of all lenses ma¬ 
nufactured for optical purposes. These properties 
eminently adapt them to cases of presbyopia and hyper- 
met ropia. But there are some defects in eyes where 
pebbles should never be used; near-sighted persons are not 
benefitted by them, nor people in need of cataract lenses, 
for obvious reasons. They are too glary for a near-sighted 
eye, and will show double refraction in thick, heavy 
cataract-lenses. Eyes, sensitive to light, should also ab¬ 
stain from using pebbles; only light smoked lenses, and 


- 84 ~~ 

iu some special cases, light blue ones can be used with 
satisfaction. The light tint of the blue ray has no dis¬ 
agreeable effect, and is almost indifferent to the feeble 
eye; while the increase of the other colors of the spec¬ 
trum becomes injurious in the course of time. 


CHAPTER IV. 


Definition of Spherical, Prismatical and 
Cylindrical Lenses. 


The word spherical is derived from sphere (globe or 
ball), and a spherical lens is the segment of a sphere. 
The size of the ball indicates the strength of the lens. 
For instance, the ball is of 2" diameter, then the segment 
is of a 2 inch focus, or as we write it -f 2. 
































- 36 - 


The lens of the first figure is plano-convex, and its 
focus is at the end of the diameter: it is cx No. 2 (two 
inch focus). The lens of the second figure is double cx, 
and its focus is at the end of the radius, or at the centre; 
its strength is + 1 (one inch focus). 

This rule is good for lenses of any other number, be¬ 
cause the relative strength of a lens is constituted by the 
curve alone, and not by the thickness or. thinness of the 
material of which it is composed. The two opposite 
curves can be widened by several plane glasses, put 
between them, without altering the focus, provided we do 
not alter the place or position of the lens nearest the 
focus, and only widen the outside half of the double 
lens. For instance, take two + 8 inch periseopie lenses, 
put the flat sides together, and measure this double lens ; 
you then have a lens of focus + 4 inch. Hold the inside 
lens steady and remove the outside lens \ inch, and the 
focus is not visibly altered . 

When we hold a spherical lens vertically in our left 
hand, and, with our right fingers, turn it around its 
centre, without moving our left hand, we see no change 
in the object we are looking at. The movement around 
its centre has no action on the object seen through the 
lens ; it is the same as if the lens were held steady, and 
not moved at all. This should be remembered, because 
it is essentially different from the action of prisms and 
cylindrical lenses. 

Manufacturers of optical instruments make use of this 
peculiarity in testing the correctness of lenses. They 
glue them upon a chuck of the turning lathe, and place a 
light at some distance in front. If the lens is well cen¬ 
tred, the light will appear in the lens perfectly steady 
when the lathe is set in motion; otherwise a light circle 
will be visible in the lens. The larger the circle is, the 
more the lens is decentred; and it is only after its centre 


- 37 - 


has been correctly determined, that the workman finishes 
the edges. All lenses of opera-glasses, telescopes, etc., 
receive their finishing touch in this way on the lathe. 

Prismatical lenses or Prisms are wedge-shaped pieces 
of glass, which break the straight line, so that it appears 
more to the thinner end of the glass, when we hold it 
horizontally over a vertical line, thus: 



The thicker end of the prism is called the^ base; and 
“ base in ” means to place the thicker end towards the 
nose-piece of frame. In setting such lenses, care should 
be taken that a straight line is not broken in either of the 

C 1 

lenses. 



If you take two prisms of the same strength,- and lay 
them so together that the thick part of one covers the 
thin part of the other, you will have a plane glass; one 
neutralizes the action of the other. . The peculiar action 
of a prism consists in the displacement of an object seen 
through it. The object never appears where it really is; 
•it is seen higher or lower, or more to the right 
.or left than it should be. This is due to the different 













— 38 — 


positions in which the prism is held. Mark again the 
difference between a spherical lens and a prism. 

We have seen in Chapter I, the manner in which we 
measure spherical lenses; let us see now how we deter¬ 
mine the strength of prisms. There is no collecting or 
dispersing power in a prism, as we have found in cx or 
cc lenses; its action is limited to the displacement of an 
object towards its thinner part. You can easily compre¬ 
hend this action by simply comparing it w T ith that of a 
cx lens. All rays falling upon a cx lens are bent in the 
direction of its centre or thickest part, so that they unite 
sooner or later in one common focus according to its 
focal strength. The rays which fall on the surface of a 
prism are also bent towards the thicker part of it, and 
follow this direction after they leave the other side of 
the prism. If our eye meet such a refracted ray of 
light, we do not see the object where it really is. 



3 


A B C is a prism, D is a ray of light falling on it, and 
is bent off in the direction of O. If our eye is placed at 
O, we see the object D, from which the light is emitted, in 
the direction of O E; and although the ray of light is bent 
towards the base of the prism, it comes apparently from 
the thinner part of it. 

Prisms have no focal power; they cannot be measured 
by the inch or metrical system; but their strength is de- 



— 39 — 


termined by the angle A B C. The opening at B con¬ 
fers upon the prism its strength and name. We have 
prisms of 1 ° , 2 ° , etc. The following figure represents 
a prism of 45 ° , or the 8th part of a circle, which is, as 
everybody knows, divided into 360 ° . 


A 



With a “ trial box,” containing test prisms, the 
strength of one lens may be determined by neutralizing 
it by another, as I have shown before; but to ascertain 
also the correctness of your test-prisms, it is necessary to 
construct a tool made of a protractor, like the following. 



The joint B must be exactly in the centre of that semi¬ 
circle DAE. That side of the rivetted or stationary 
bar A B which is nearest E, is precisely 90 ° from either 
D or E. The arm B C is movable, and indicates the num- 











_ 40 — 


bers of degrees of a prism placed in the opening A B C. 
I present also another easy way of testing prisms by 
the use of a simple ruler. 



Take a ruler of 12", American measure; cut a notch 
in the middle of its edge (at 6 inches) large enough for 
the reception of the base of the prism. Place one 
-end of the ruler on the line A B, and the other 
on the - cheek-bone, just below your eye; lay the 
base of the prism in the notch, so that the line A B is not 
broken in the prism ; then see how far towards the right 
the line C D is displaced, and you will find that each 
degree represents 1/16 of an inch; a prism of 16 ° dis¬ 
places the line C D, therefore, exactly one inch. 

Cylindrical lenses are ground and finished with a cylin¬ 
der, instead of the segment of a ball used, as we have seen 
before, in grinding spherical lenses. If the outside of a 
cylinder is employed, the lens will be concave-cylindrical; 
and when the concave side of a section of the hollow 
cylinder is used, the lens will be convex-cylindrical. The 
only difference in finishing either a spherical or cylin¬ 
drical lens, say of 5 inch focus, consists in the first case that 
the grinder has to take a segment of a ball of 5" diameter, 
but for a cyl. lens of the same denomination he has to 
take a cylinder of 2J" diameter, or double the strength, 
as the number of the lens he wants to grind, indicates. 












- 41 



4 he axis of such a lens passes along the highest or 
lowest ridge of it, and is easily determined by moving 
the lens up and down, and finding by its gradual turning 
that line where there is no action at all. So long as the 
object seen through the lens, moves with the motion of 
the lens, the axis is not yet found. 

Cylindrical glasses have quite a different axis from the 
spherical, which possess, as will be seen in the next 
chapter, a common centre, from which the optical lines 
radiate in all directions of the compass, while there is 
only one optical line or axis in a cyl. glass. Objects seen 
through this line are either lengthened or shortened ap¬ 
parently, which is best demonstrated by looking at a 
square. 



A cx cyl. lens with axis vertical will lengthen the hori¬ 
zontal sides, producing a horizontal parallelogram. A cc 
cyl. lens will have the same effect, with the difference that 
the lengthening is in the direction of the axis, and if this 
is vertical, the parallelogram will be vertical. This ex- 
4 




















— 42 — 


plains why a cx and cc cyl. lens, laid together, axis upon 
axis, will counteract each other, and restore the parallel¬ 
ogram to a perfect square. When you take two cyl. 
lenses of the same strength, and place the axis of one 
vertically, and of the other horizontally, you destroy all 
the cyl. action, and retain only the strength of a simple 
spherical lens of the same number or strength as that of 
the cyl. lenses. Take, for instance, two lenses of—2 C, 
lay their axes crosswise, and you have —2 S which is 
neutralized by + 2S, thus producing a plane lens. This 
is a simple way to find the number of cyl. glasses, when 
not marked by the grinder. 

Some thirty years ago, a French optician, Galland de 
Chevreux, introduced such cross-cyl. glasses, claiming 
that they obviated that small degree of incipient astig¬ 
matism with which nearly every eye is afflicted. A careful 
comparison of them with spherical lenses will show the 
fallacy of this claim. This, and their high price, have 
brought them into disuse. 

Cross-cylindrical lenses are yet used for the correction 
of aggravated and complicated astigmatism, but then 
they are always of different denominations; they are cx 
on one side and concave on the other, both of different 
angles according to necessity. 

To determine whether a lens is cx or cc cyl., which is 
sometimes doubtful among those of lower strength, we 
have only to look through them at a vertical line, 
and move the lens to the right or left. If the line moves 
in the opposite direction of our motion, the lens is cx; 
but if the line follows, the lens is concave. 

Generally, the grinder marks the axis by small lines at 
the border of the lens. 

The use of cyl. glasses has increased lately to such an 
extent, that no optical establishment comes up to the re- 


— 43 — 


quirements of the trade without being able to fill correctly 
the orders of oculists. One tenth of all eyes are more or 
less astigmatic; and since oculists took the selection of 
spectacles in hand, the demand for cylindrical glasses is 
very great. 


CHAPTER Y. 


Optic Line and Centre, and How to Decentre Lenses. 


Some twenty years ago, a traveler for a New Pork 
manufacturing house offered to me spectacles for sale 
which he called “ Perfected,’' and when asked what he 
meant by it, said that the lenses were correctly set, the 
frames well tempered, and the whole spectacle perfect. 
To my great surprise, one glass of the first pair I ex¬ 
amined, was badly centred. He excused himself by say¬ 
ing that he was not an optician, that he only represented 
the goods according to instructions given him by his em¬ 
ployers, and promised that ail goods I might order 
through him, should be without any fault whatever. He 
admitted further, that no member of his firm was a prac¬ 
tical workman, but said that the factory was superintended 
by a competent optician. Now, if this foreman really 
understood the meaning of an optical centre in a lens, 
why did he not instruct the glass-setters how to be exact 
in the fitting of lenses to the frames, especially of those 
“ Perfected Spectacles,” for which they charged $4.00 a 
dozen more than for other goods of the same style and 
quality? I know not whether this name was invented 
only for the sake of extortion, or whether they charged 
so much more for the stamping of the temples, which 
was indeed nicely done in gold letters. 

To be able to readily determine the optic line of a lens 
is more important for an optician than any other ac¬ 
quirement of his trade. It is the essential requisite for the 
correct manufacture of all optical instruments, — spec¬ 
tacles, opera-glasses, telescopes, or microscopes; the 




— 45 — 


optical centre must have its right place and position, or 
the instruments will be incorrect and worthless. 

The best way to find this centre is to look through the 
lens at a well marked straight line—drawn with pen and 
ink and a ruler across a sheet of paper. Hang this pa¬ 
per against the wall some four or more feet from you. Then 
take the lens between the thumb and first finger; extend 
your arm, shut one eye, and look with the other through 
the lens at the line. You will observe that the line is 
broken in the lens, and the more so, the nearer you move 
the lens towards its border. Figure a and b. 



Now, move your lens slowly towards the centre till 
you find the line unbroken (Fig. c) ; mark this line with 
ink, and it will be the optic line of the lens in one direc¬ 
tion ; but you have not yet determined the optic centre . 
Now turn the lens in your hand 90 ° , so that the line on 
the paper and the mark on the lens form a right angle. 

Proceed in the same way as before, and you will find, 
very often, that the optic centre is not always in the 
middle of the lens, Figure cl. 












— 46 — 


The two lines should cross each other in the middle of 
the lens, as they do in Figure e. 

This test will do for spectacle lenses; but the test for 
scientific instruments is more elaborate, as we have seen 
in Chapter IY. 

I would advise you now to take at random a dozen of 
spectacles from your stock in trade, and examine them 
as to the correctness of their optical centres. You will 
find that many of them, highly valued in the market on 
account of their trade-mark, are grossly incorrect, and 
good for nothing. It will cease to be a matter of aston¬ 
ishment that some of your customers could not see with 
one pair of spectacles, and yet found others of the same 
number pleasant and satisfactory. 

To decentre a lens is an easy task for any one who un¬ 
derstands the nature of the optic centre. As we have 
seen in Chapter IY, under the heading of “ Prism,” that 
we have to set them either “ base in” or “base out,” it 
is sometimes necessary, in order to overcome certain de¬ 
fects of vision, to decentre spherical lenses, and cause 
them to act like weak prisms. To fill such an order cor¬ 
rectly, it is necessary to first mark the optic centre on 
the lens, then put the zinc-pattern (Chapter YI) as much 
as possible to one border of the lens, and make your 
mark around the pattern. 


M'-A- 





That border of the lens nearest the centre, is the base; 
and any order of “base in” or “out” is correctly 
filled, if you place this part (/) towards the nose or tem¬ 
ple, according to order. 




CHAPTER VI. 


Fitting or Spectacle-Glasses, Their Filing and 
Drilling. 


This chapter is the continuation of the foregoing, 
which explained the mam points, concerning the correct 
fitting of glasses. We work blindfolded when we are 
unable to find the centre of a lens, and it will be by 
mere chance, if our work is correct. Rough lenses are 
not always well centred; if they were, we would have 
simply to cut the size we need from their middle, and 
there would be no mistake. Many of them will be found 
so much decentred as to be useless for size 0 and 1, and 
only fit for size 3 or 4, or they may be altogether worth¬ 
less.* In a well centred lens the edges are equally thick 
on their opposite borders, and a little practice will enable 
the eye to see at a glance, or even without looking 
through it, whether the lens is decentred or not. This 
saves us a good deal of time, as the principal test is then 
quickly determined. But we should not rely altogether 
on the judgment of our eye in this regard, as it requires 
a good deal of practice to detect small differences in 
weak lenses. 

Any workman with good tools can perform in a short 
time more and better work than others who shufile about 
the whole day long, wasting time and material, for want 
of proper implements. The most useful tool in setting 
glasses is the model or pattern , made of thin zinc. If 
you have not yet made use of them, prepare a set of the 


Except for lenses to be decentred. 





— 48 — 


different sizes and shapes of spectacles, as these come 
into your hands for repairs, and mark them according to 
the different sizes of the eye. Make a hole exactly in 
the middle, partly for purposes to be spoken of in the 
next chapter, and partly to suspend them on the wall 
within convenient reach, well assorted according to size 
and pattern. About three dozen will fully assort you, 
and will save you, in the course of years, an immense 
amount of trouble and time. 

Another important tool is the marker , an instrument 
like a lead-pencil, mounted at one end with a small dia¬ 
mond. The marker is used to make a scratch around 
your pattern, after it is placed correctly on the lens. It 
will not cut the glass as a glazier’s diamond, because it is 
intended only for scratching purposes, and is, therefore, 
very cheap. On heavy lenses it is best to mark both 
sides, to prevent the breaking of the lens inside the 
mark. 

The next tool for our purpose is the sliding-tongs, an 
instrument employed by watchmakers and jewelers, who 
call it the “ dog nose sliding tongs;” it is also used by 
opticians to chip the lenses. 



I have found the largest size the best for almost all 
lenses; but very thin glasses can be chipped better with 
common flat pliers, as, for instance, glasses for lockets 
or watches, which you may occasionally be obliged to 
grind. The apprentice should practice this chipping 
well on pieces of window glass, before he attempts to 
shape a good lens, and spoil it, perhaps, by inexperience 
in handling the tool. 




49 — 


The proper way to handle this tool is the following: 
The tongs, held by the right hand, should be applied 
loosely to the lens, and worked as we do a pair of scis¬ 
sors, with the difference that at the same moment we 
close them, we also give the upper part of the tongs a 
slight inclination to the outside and downward. The 
lower nose is kept right on the mark by the middle lin¬ 
ger of the left hand. This effectually prevents the lens 
from cracking inside the mark. The outside movement 
of the tongs throws the chips and glass-splinters from us, 
and thus saves the eyes from injury. But a fine glass- 
dust also rises from the lens, and is very pernicious to 
the lungs. Hold the lens, therefore, nearly at arm’s 
length, and blow the dust off before you breathe. 

As a rule, we should move the tongs outward; but we 
may come to a place which will not break readily, even 
by applying greater force. In this case we can some¬ 
times accomplish our task with ease, and without the 
risk of spoiling the lens, by moving the tongs upwards, 
using the lower nose for the breaking, and the upper as 
a guide. This alternate turning up or down of the 
tongs should be well practiced by the apprentice. 

It is hardly necessary to mention here that the stone 
has to be turned from you when grinding. I have seen 
only one jeweler (and he, too, styled himself “opti¬ 
cian,”) who turned the stone to him, as he had seen 
done by a street-grinder. Is it to be wondered that he 
complained afterwards of not being able to get a smooth 
edge on his glasses, or that they looked as if rats had 
given them the finishing touch ? 

I do not think it out of place to say here a few words 
in general about the grinding of lenses. Almost all 
manufactories grind them into a sharp bevel, which is in 
my opinion an unnecessary trouble, and, besides, shows 
very little sound judgment. The grooves of most frames 
5 


— 50 — 

are not pointed, but rounded off, whether they are made 
of soft material or metal; and the lens, to properly fill 
such a groove, should be also rounded off. This will 
have the double advantage of being less liable to crack, 
and less troublesome to finish. Sharp-pointed lenses 
easily split shell or rubber frames, when the latter con¬ 
tract in cold weather; or they themselves are chipped by 
metal frames, when they are tightly fitted. To over¬ 
come this difficulty, and to establish a practical method 
of fitting lenses to the frames, I will describe the method 
which I adopt. After the lens has been well shaped and 
sufficiently reduced by the sliding-tongs, I grind off the 
sharp edge on one side by passing it quickly over the re¬ 
volving grinding-stone. A few revolutions will accom¬ 
plish this, and will give it a small but distinctly visible 
bevel. Then I do the same with the other side, by turn¬ 
ing the lens alternately edgewise, to take away its un¬ 
evenness. In less than one minute my lens has a finished 
appearance, and needs now only the final adjustment. 
The*edges of the lens have then a rounded form, and 
when set in frames do not show any roughness, as the 
polished surface of the lens touches the border of the 
mounting, thus relieving me of the trouble to polish the 
bevel, which, however, cannot be avoided when the 
lenses are thicker than the frames, or when the grooves 
are very shallow. 

In regard to the present universally adopted habit of 
polishing the edges of lenses, I must confess that I do 
not approve of it, for the good reason that the reflected 
light from such bevelled surfaces is very annoying to the 
eyes, and can be easily removed by giving them only a 
fine ground finish, which the German and French call 
“matt.” Even frameless spectacles could be made in 
this way, and would look equally stylish. But this re¬ 
form can only be effectually introduced by the unani- 


— 51 — 

mous co-operation of the oculists in rejecting in future 
all glasses with polished edges. 

The fitting of bevelled glasses into the groove of the 
frames is quickly done, and they are easily ground and 
shaped if they are of an oval or round pattern. Octagon 
glasses require more attention, especially when the 
frames are old and often repaired. The greatest care 
has to be taken with skeleton and grooved glasses, as the 
edge should be flat, and the bevel very small. The stone 
should be used till the lens is rightly shaped and the edge 
roughly flattened; w T e should then finish the lens on 
emery paper Nos. 3 and 2, and lastly on No. 1 and 0 for 
polishing purposes. If the lens has to be grooved, No. 3 
is used only for the edge, but Nos. 2 and 1 for the 
bevel. It is better to finish the bevel before filing: the 
groove, as a polished surface is less liable to chip in case 
the file should touch the edges. The grooving is always 
done with a round file., never with a four or three-cor¬ 
nered one. The file will soon be smooth if used dry; it 
is therefore necessary to wet it constantly either with 
water, turpentine, benzine, or dilute sulphuric acid; the 
latter is most effective. But even these will generally 
ruin the file after the finish of one pair of lenses, thus 
considerably increasing the cost and labor. The best 
fluid for the preservation of the file and drill for our 
purposes, is one that contains an excess of camphor. 
Any mechanic knows that a new file should not be used 
at once for filing hard iron or steel, without passing it 
first several times over a softer material as wood, brass 
or soft iron, to fill up the deeper parts of the file, giving 
strength to the exposed sharp points of it. Camphor 
renders the same service to our file used for grooving 
glasses, without interfering with its cutting qualities, if 
the fluid evaporates quickly enough to allow the camphor 
to clog up the deeper parts of the file. To do this by 


- 52 — 


passing it oyer lead, would cause it to slip without cut¬ 
ting the glass. The formula for this fluid is: 


Spirits of Turpentine.1 ounce. 

Camphor Gum.1£ “ 

Sulphuric Ether... .3 drachms. 


The ether facilitates the solution of the camphor, 
and then greatly volatilizes so quickly, that the file 
would be dry after a few strokes, if the turpentine did 
not retard it for a while. Keep the file, therefore, con¬ 
stantly wet while using it, and it will do service for a 
good length of time. 

The drilling or boring of glasses for skeleton or 
frameless spectacles is done by a drilling machine; but 
if you have none, it can be done also with a round file and 
this fluid. Select one almost of the size of the hole you 
need; break off the point, and commence the hole by 
moving to and fro the sharp edge of the file, previously 
dipped in the camphor preparation. Make a mark on 
the glass, then raise the file by degrees perpendicularly 
to the lens, and use it as a drill by turning it between the 
fingers. When the hole is half through, commence on 
the other side, and reduce pressure gradual^, to prevent 
a sudden advance of the file when nearly through. The 
holes are finished off by a three-cornered sinker, much 
larger than the hole itself, which bevels the edges of it, 
and prevents the breaking of the lens by the subsequent 
insertion of the screw. 

There are many devices recommended to shape drills 
for glass-boring purposes; all agree that they should 
never be pointed in the middle, but be rounded up, or 
flat like a chisel. My favored drills- were always made 
of a round file (rat-tail), by grinding off two opposite 
sides, so that it had almost the shape of a square. Then 
holding the file at an angle of 60 ° , I smoothed the lower 





— 53 - 


surface with the oil-stone, forming a slanting plane, and 
producing a sharp strong edge to cut with. Another 
good drill is made of a three-cornered file, sharpened in 
the usual way, but with one corner taken off, so that the 
cross section of the drill near the point is that of a trun¬ 
cated cone, and the end of the drill of a narrow chisel- 
shape. 

Not all files make good drills. Either they are not 
well tempered, or the grain of their steel lacks that pe¬ 
culiar cutting quality, which'we find again in others. If 
you see, therefore, that your drill does not cut readily, 
throw it aside and try another file, till you find one that 
works well. I have often rehardened them, but gener¬ 
ally without success; the steel was not precisely of that 
quality which is necessary to make a good drill. When 
you have secured such a file, take a jealous care of it; I 
have used some for years, and found them always reli¬ 
able like old trustworthy friends. 


CHAPTER VII. 


Measuring of Compound Lenses, and Their 
Correct Setting. 


Simple defects in the refraction of eyes can be cor¬ 
rected by spherical cx or «c glasses; and when their 
right number or strength is selected for each eye sepa¬ 
rately, and afterwards correctly set in suitable frames, 
such spectacles will always give full satisfaction. Nine- 
tenths of those in need of glasses are well suited with 
simple spherical lenses, and can be rightly served by the 
optician as well as by the oculist, who, if he is never¬ 
theless consulted by over-anxious people, can do no more 
than we do: he uses his test-types to find the extent of 
the error of refraction, and selects the spectacles ac¬ 
cordingly. But others require something more than an 
optician is able to do; these should be sent to an oculist, 
who, after a professional examination, will give his or¬ 
ders, and, generally, for compound lenses. 

Compound lenses are combinations of spherical, pris- 
matieal and cylindrical glasses, of which two, or in some 
cases all three, are ground on one and the same lens. 
The most simple combination is when the plane sides of 
a prism are ground into the spherical shape of either cx 
or cc, without altering the action of the prism. 



An order for such a lens will read for instance: 
+ 3 S Q prism 2 ° , or perhaps: — 2S Q prism 3 ° ac¬ 
cordingly. 







— 55 — 


The combinations of compound glasses are so mani¬ 
fold that they have to be ground always to order, as no 
optician can have them in stock. We should never rely 
on the faithful execution of our orders by the grinder; 
we may have copied it indistinctly, etc. It is therefore 
advisable to remeasure all lenses before we fit them to a 
frame. Let us take the first lens as a test. We have 
here a spherical + lens combined with prism of 2°. I 
suppose that each of my readers has a trial-box with all 
the different lenses; if he has not, he should procure 
one as soon as possible, for no optician can do without it. 
We first take from our box a prism of 2 ° and place the 
thick end upon the thin one of our lens. We will see at 
once that the optic line, which was before near the bor¬ 
der, is now in the centre. We then take — 3 S and place 
it on the other side of the lens; these three together 
must now be plane, or the lens is not correct. 

The next combination is the sphere with a cylinder . 
One side of the lens is ground spherically, the other 
side, cylindrically. Such an order reads: +2.5 S O 
+ 1.25 C. The test is the same as before. With — 1.25 G 
we neutralize the cylindrical action in this lens by lay¬ 
ing the two axes so as to cover one another perfectly, 
and by adding — 2.5 S we must again have a plane lens. 
The grinder always marks the axes by little scratches 
upon the edges of the lens, so that we have no trouble 
in measuring them. 

But how, when the grinder forgot to mark the lens, or 
when we are compelled to find the formula of compound 
spectacles, having no mark, handed us by a stranger to 
duplicate them ? 

Let us first see if they are decentred; if so, we will 
find one side of the lens thicker than the other, or that 
a horizontal line is elevated or lowered by turning the 
lens between the fingers to the right or left. When this 


- 56 — 


is the case, they are combined with a prism. We neu¬ 
tralize this prismatic action by trying different degrees 
of prisms till we get the optical line in the middle, or 
till there is no more breaking of the horizontal line. By 
turning the lens now, the optic line will not move up or 
down,"as it did before the prismatic action was neutral¬ 
ized. 

Keeping these two lenses in position, we notice 
whether the cyl. part of the lens is cx or cc, by moving 
it to the right or left in front of a vertical line (Chap¬ 
ter III). When this line follows our motion, the lens is 
concave, and has to be defined by a cx cyl. lens, The 
remainder of our lens is simply spherical, and easily 
measured. To prove the measurement, and especially 
to determine the right position of the angle of the cyl. 
part, we first neutralize the prism, and then the sphere, 
and lastly find the axis of the cyl. paid by following the 
rule given in Chapter III. We now mark this line with 
pen and ink, and place our lens on the following figure: 











— 57 — 


The centre of the lens must be exactly over the cen¬ 
tre of the circle, and the horizontal line marked on the 
lens from nose to temple must cover the horizontal line 
AB. We now observe in what direction the ink-mark 
points, and we have the degree of the cyl. axis. In 
making this proof we must hold always the outside of 
the lens upwards, not towards the paper. 

All measurements of the physician refer to the posi¬ 
tion he takes toward the patient, his right is the patient’s 
left. 

I have made for my own use this delineation on strong 
paste-board, covered with white paper, and find it very 
handy and more accurate than anything I used be¬ 
fore. The little lines are useful guides for finding 
the right position of the zinc-pattern, and dispenses 
with the labor of searching for the true centre through 
its hole. 

Now, my young friends, I have tried to explain this 
matter in as few words as possible, and in the most 
practical way; but some of you may think it too compli¬ 
cated a task, and lose confidence in your own ability to 
overcome certain difficulties. Just try it, and if it takes 
you a whole hour to measure a compound lens over and 
over again, you will laugh at this “Sphinx” after¬ 
wards, when you will be able to solve the problem in five 
minutes. 

It is absolutely necessary to know well how to meas¬ 
ure compound lenses before you can be able to set them 
correctly. I admit that there are difficulties which will 
puzzle the inexpert, and will lure them into a different 
calculation altogether. I give here a few illustrations: 
We have, for instance, a lens — 0.50 S Q — 1 C ax 90 ° , 
the formula of which we do not know. By looking 
through the lens we see at once that the concavity is in 
excess of the convexity, if there is any at all. We first 
6 


— 58 


look at a vertical line, and notice whether it will follow; 
if it does, and we pursue our investigation and correct 
the cyl. action by a suitable cx cyl. lens, we are on the 
right track. But if, perchance, we had turned the lens 
i of its circumference, and had examined it in this posi¬ 
tion, which is at right angles of the true cyl. axis, we 
would have found that the vertical line did not follow, 
but acted as a cx cyl. lens, and we would have had to 
make the correction in this case with a concave cyl. 
lens. The consequence would be that we had made 
cross-cylinders (Chapter III), and adding — 1 S to 
the — 0.50 S, which was before the real amount of 
its spherical concavity, the formula found would be 

— 1.50 S Q + 1 C ax 180 ° . In order to detect the mis¬ 
take, all that is necessary is to try the lens with + 1.50 S, 
and we will see at once that we are wrong, because our 
lens of only — 0.50 S will not be neutralized by the 
test-lens 4* 1.50 S. 

For another test let us take — 0.50 S 3 + 2 C ax 
180 ° . The cyl. axis is easily detected in such a lens by 
its shape; but for argument’s sake I suppose we have 
fallen into the same error, and again produced cross-cyl¬ 
inders, thus turning + 2 C into + 2 S. Then we have 
to add -f 2 S to — 0. 50 S, which would give + 1.50 S, 
and our formula would be + 1.50 S 3 — 2 C ax 90 ° . 
If now we take our lens and cover it with a test-lens of 

— 1.50 S, we shall see our error clearly. 

We take another lens: 4- 1.75 S Q | 0.50 C ax 
90 ° , and by the same faulty process we will get + 1.25 
SO — 0.50 C ax 180 c ; but by covering our com¬ 
pound lens with a test-glass of — 1.25 S, we perceive 
that we have made a gross mistake, and have to recom¬ 
mence our investigation 90 ° from the line we mistook 
for the true cyl. axis. 

Such an incorrect lens would be utterly useless to the 


59 


patient for whom it was prescribed, because it is in¬ 
tended to correct where there is no correction necessary, 
and leaves the really impaired axis of vision unim¬ 
proved. It is, therefore, well to measure compound 
lenses first in one direction and afterwards 90 ° from 
that line, when it will be an easy task to find the true 
axis of the cyl. part of the lens. In order to find the 
formula, the axis should always be marked by small 
scratches at the border of the lens, or by pen and ink on 
a lens already fitted. 

The fitting of a compound lens to a frame next lays 
claim to all our attention, if we will do justice to the 
general rule, i. e., to bring the spherical centre before 
the pupil of the eye. When we have marked the cyl. 
axis in ink across the whole lens, and have neutralized 
the cyl. action by its opposite, we must next observe 
where the optic line crosses the lens 90 ° from the cyl. 
axis, and mark it likewise in ink, but in a manner differ¬ 
ent from the line of the cyl. axis, say by little dots. 
We now lay that point of our lens where the two lines 
cross, exactly over the centre of the test-figure, turn the 
axis of the cyl. to the prescribed angle, and mark by lit¬ 
tle scratches at the edges of our lens where it touches 
the horizontal line A B. These marks are guides to di¬ 
rect us in regard to the nose-piece and temple. We must 
take care that the hollow side of the lens lies upon the 
paper, because that side will be towards the eye. Our 
zinc-pattern, after which we mark the lens, must have a 
hole exactly in the middle, and a marked line from the 
nose-piece to the temple. 







— 60 — 


Through the hole we can see the point where our ink- 
marks cross; we put the line of the pattern so that its 
continuation strikes the scratches made before as a guide 
for the nose and temple, and after ascertaining once 
more by careful examination that everything is right, we 
continue our marker around the pattern. Before chip¬ 
ping off the superfluous part of the lens, we take a small 
wooden ruler, place it on the lens, touching the two 
marks for nose and temple, and make two other fine 
scratches inside the mark just made for the size of the 
lens, long enough not to be ground away in the finishing 
process. After the chipping, we have only to pay at¬ 
tention that our lens retains a nice oval shape, and that 
the edges are well bevelled. 

Any optician who follows these instructions cannot 
fail to give full satisfaction to the most exacting oculist, 
no matter at what angle the axis of the lens has to be 
placed. I believe that some opticians are careless in 
marking the true centre of the lens, and use, to find the 
angle, designs similar to the following: 



I republish this cut as a sample of the incorrect man¬ 
ner in which they are generally made, and to guard 
against their use by any jobbing optician. 

In the 4 ‘Jewelers’ Circular and Horological Review” 
of November, 1885, I find on page 312 the strange com¬ 
plaint of a well-known oculist, saying: “You will seldom 




— 61 


find a workman who can* exactly set a cylindrical lens at 
the axis required, unless the axis named be 180 ° or 
90 ° . You will probably have to tilt the frame a little, 
either up or down, to obtain the exact position required. 
That they set more lenses wrong than right, has been my 
experience.” 

If his opticians use the above design to find the angle, 
their lenses must, of course, be incorrectly set, except 
in those two directions, as they are the only correct 
ones. It is, therefore, no wonder that the “Doctor” 
finds fault with his opticians. 

It may be deemed by those of my readers who, by 
reason of their education and long experience, are famil¬ 
iar with both the science and art of our business, that I 
have rendered myself tedious by entering too minutely 
into details, and being less concise than the subject de¬ 
manded. While hoping that no one may fail to find 
something to repay him for the perusal of my book, I 
beg to say that these pages are directed not only to the 
proficient, but also, and perhaps more so, to the begin¬ 
ners in our difficult trade. I have aimed to elevate them 
to the full requirements of our occupation; with this 
view I have presented some things which might other¬ 
wise have been omitted, and, to be well understood by 
them, have been, perhaps, in some places, more than or¬ 
dinarily diffuse. 


CHAPTER VIII. 


Selection of Spectacles: Pupil Distance, Nose- 
Bridge, Reading and Street Glasses. 


This chapter is written only for young opticians and 
such persons as have not yet acquired sufficient experi¬ 
ence in the selection of spectacles, to overcome the 
many vexations incident to their particular trade. 

The two essential parts of spectacles are the lenses and 
the frames. It is not my object here to give a treatise 
on the selection of the proper lenses for the correction 
of the many deficiencies and irregularities of the eyes, 
as this would compel me to enter into their anatomical 
construction and their defects, which can be found ably 
described in the many medical books published for this 
purpose by eminent physicians. My object is to give an 
inside view of the practical and mechanical part of our 
trade, and leave the theoretical portion to “ Specialists.” 

I must refer first to the pupil distance , as this is the 
main point of a good fit of a pair of spectacles. Pupil 
distance is the length between the two pupils, measured 
from the middle of one to the middle of the other. This 
distance is never smaller than two, nor larger than three 
inches. The eyes of little children, as well as those of 
the largest men, are within this compass. The average 
pupil distance of a grown person is 2f or 2} inches, and 
these are the standard sizes the manufactories use for 
most spectacles they make. An optician is, therefore, 
obliged to have for any emergency an assortment of all 
the different widths ranging from 2 to 3 inches. Chil- 




— 63 — 


dren require 2" and 2J"; boys and young girls 2J" and 
2§"; grown persons with small faces use mostly 2§". A 
full face needs 2£" and 2§". Near-sighted people have 
generally a large pupil distance, and very often require 
as high as 2f inches. I have had in my extensive prac¬ 
tice only three customers whose pupil distance reached 
fully 3", and all of them were near-sighted. I myself 
use 2J", and share the same fate; I, too, am near¬ 
sighted. 

An ordinary dealer has a fair assortment with spec¬ 
tacles or frames from 2:J" to 2f", most of them of 2f" 
and 2£" pupil distances. 

A simple way of finding the size of spectacles is to 
measure the length of the nose-piece and one eye , which 
gives exactly the true pupil distance: 


a ___ 4 



because if you shift the line a h to the left, so that a 
is vertical to a', then h will be vertical to 6', which are 
the true centres of the frame. 

Another important point is the selection of a proper 
nose-piece. People with a low or shallow bridge should 
not, or rather cannot, wear eye-glasses; and even spec¬ 
tacles of the ordinary size are not satisfactory, if the 
nose-piece is not shaped so as to correct the deformity of 
the nose. Formerly there were only three nose-pieces in 
use, the C, K and X, to which lately have been added 
the snake and saddle nose-pieces. 







- 64 - 



X 



Snake. 


Saddle. 


The X and snake nose-pieces are the best for low 
noses and street glasses; the last is especially useful in 
removing the glasses far enough from the eyes to free 
the eyelashes from coming in contact with them. The 
only objection to most of these nose-pieces is that they 
are rather thin, and consequently cut the nose, if the 
skin is tender, as is the case with children and ladies. It 
is strange that means are sometimes employed to remedy 
one evil by the substitution of another. Instead of 
making the nose-piece broader, Dr. Hubbell invented a 
nose-guard, a broad attachment to the nose-piece, which, 
of course, prevents the cutting of the nose, but at the 
expense of its look. 



If the nose-pieces were made sufficiently broad, well¬ 
shaped and polished, they would not need any lining of 
turtle-shell or cork, such as are made lately at some ad¬ 
ditional expense, while the broader nose-pieces, answering 
the same purpose, can be manufactured at the same cost 
as the thin ones now in use. 


— 65 


Beading spectacles should always be in such a position, 
as to permit us to see through the middle of the glasses 
without being obliged to bend our head down or for¬ 
ward. We should be able to see at an angle of 45 ° 
through the middle of the glasses with our head straight, 
and by merely lowering our eyes in that direction. These 
glasses must be placed considerably lower than the 
street glasses, which, on the contrary, enable us to see 
through the middle of the lens when looking straight 
ahead. The military rule for this position is that the 
eyes should strike the ground at forty steps from us, 
which is about one hundred feet. Near-sighted persons 
should be fitted in this way. It looks very bad when the 
street glasses sit too low, and oblige the wearer, in order 
to see through the glasses, to throw his head back, as if 

he were star-gazing. 

© © 

In regard to this stooping position for working pur¬ 
poses, I may mention here the reason why people should 
not bend their head forward, but keep it erect while 
reading, etc. Any medical book will inform you that 
the arteries which carry the blood from the heart to the 
head and all the parts of the body, are situated far be¬ 
neath the surface, and that those blood-vessels which you 
can see just below the skin are veins which conduct the 
blood back to the heart. Now feel the muscles of your 
neck when erect, and again when stooping; they are soft 
and pliable in the first position, and hard and stiff when 
you bend your head forward. The arteries being situa¬ 
ted below the muscles, their action is not influenced by 
the changes of the latter from the contracted to the re¬ 
laxed condition; but the circulation in the veins is con¬ 
siderably interrupted by their being compressed to a 
much smaller size than before. What is the consequence? 
The pumpjng of the blood into the head goes on unin¬ 
terruptedly; but the flowing off to the heart is ob- 
7 


66 — 


structed, and we sooner or later suffer from headache, 
get dizzy, and have to stop work. How many times is a 
spectacle-dealer puzzled by such complaints of his cus^ 
tomers, not knowing how to correct it? Trying in vain 
stronger numbers !!—till at last these people by chance 
find among the cheap, common spectacles a better fitting 
frame , and, of course, also temporary relief. We not 
only lose this ill-pleased customer, but drive him to the 
conviction that twenty-five cent spectacles are just as 
good or better than two-dollar ones. We are the cause 
of his at length ruining his eyes by the use of these 
common spectacles, through our ignorance of the nature 
of his reasonable complaints. Direct, therefore, great 
attention to the fitting of suitable frames. 

It is hardly necessary to mention that it is absolutely 
necessary to examine both eyes separately, and to cor¬ 
rect any error of refraction by the proper lens. But 
there are cases beyond the sphere of opticians, i. e., 
when it is impossible to make the right diagnosis with¬ 
out preparing the eye for such an examination. These 
patients should be turned over to an oculist; it 
would be an act of “ charlatanry ” on our part to pre¬ 
tend to do full justice to such cases. Confine your skill 
to the limits of your trade, and you will be convinced 
that it requires all your knowledge, intelligence and en¬ 
ergy to fill the place of an expert optician. Over-ambi¬ 
tious young men may commit the error of trying to com¬ 
bine the two branches of an oculist and an optician as 
far as spectacles are concerned; but is it not the mistake 
of a builder who would be his own architect, the apothe¬ 
cary, his own doctor? The public in general fares bet¬ 
ter if these branches are divided, and ably represented 
by competent specialists: on one side the scientific ocu¬ 
list, on the other side the skillful optician, both experts 
in their particular branches. If we play oculist, why 


— 67 — 


should not the oculist play optician, and keep a stock of 
spectacles on hand? Therefore my advice: “Suum 
cuique.” 


CHAPTER IX. 


Double Focus. Single and Split Glasses, Relative 
to Optical Line or Centre. 


The failing of our eyesight manifests itself by the 
gradual lengthening of the focal distance. At first we 
could see well at 14"; next we are compelled to hold our 
book or paper at 15"; afterwards at 16", etc.; and the 
progress of the lengthening of our focal distance, slow 
at first, soon takes a wonderfully rapid stride, if we hesi¬ 
tate to substitute by spectacles that part of our power of 
vision which is irrevocably lost. We are reminded here 
of the common adage: “One stitch in time saves 
nine”; i. e., the early use of spectacles, when their as¬ 
sistance is necessary, saves our eyesight from its other¬ 
wise rapid failure. Nine persons out of ten, who come 
for their first glasses, confess that they have put oft the 
use of them as long as possible, but have to yield at 
last. They are not aware of the great blunder they 
made by taxing their diminished power of vision in the 
same degree as they did when their eyes were enjoying 
yet their full strength. 

A well-known fact of our “losing sight ” is the im- 
provement of distant vision; the sight is going away 
from us, we gain at the distance what we lose near by. 
Let us see fifteen years later what has become of our 
customer’s eyes and his spectacles. The gradual failing 
of his eyesight has compelled him to increase the 
strength of his reading glasses, till he uses now + 2.50 
Diopters (or + 16 inch focus) for near vision; but his 
far point has also removed, and he finds it impossible to 




— 69 — 


distinguish the features of the minister, or the faces of 
people in the street 50' or 100' away from him. He asks 
for street glasses. And here arises the question: Is it 
advisable to combine reading with distance glasses? The 
most rational way is to take separate glasses for each 
purpose. Most people will follow your advice, and 
change their glasses accordingly. But we have to deal 
also with nervous, quick-tempered and impatient cus¬ 
tomers, who grumble at the slowness of steam, and will 
have everything go by lightning. They imagine that 
they have no time to change their spectacles; and in¬ 
deed some people have not. There is the accountant, 
whose entries in the ledger from the journal force him to 
look at items 4' to 6' from him. lie cannot keep a com¬ 
fortable seat and accomplish his task, if he has to jump 
up, and bend his body, and stretch his neck right or 
left, to check off and make a correct entry. There is 
the paying-teller, who must have a sharp eye on his 
money and the party receiving it. There is the engineer 
watching his engine, and looking every now and then at 
his steam-gauge; the teacher, the minister, the orator, 
the clerk, the lawyer, and many other persons, who find 
it absolutely necessary to be enabled to see well at a 
glance far off and near by. Can we accommodate these 
people without injuring their eyes? We can, with 
double-focus spectacles, each glass adapted to its special 
purpose, the upper part for distant, the lower for near 
vision. These spectacles are called “ Franklin glasses/’ 
because Benjamin Franklin was the inventor of them. 

There are two kinds in use: the double focus single 
lenses, where the upper part is ground off to a weaker 
focus, and the split glasses , where the distant ^and near 
lenses are cut through the middle (or optic line), and 
finished so that the split forms a straight line in the 
frame from temple to temple. The optic line in these 


— 70 — 


glasses is, therefore, right on the split. The wearer, of 
course, is obliged to look below or above that very line 
where the eye is most at ease, and where only it feels 
comfortable, according to facts demonstrated in Chap¬ 
ter IY. 

The double-focus single lens has another more serious 
defect. It confuses the wearer in regard to the true po¬ 
sition of things. If we look at a straight horizontal line 
first through one part of the lens, and in the next mo¬ 
ment, by moving the lens, through the other part, we 
observe at once that the line is considerably displaced. 
It is elevated or lowered as we look at it alternately 
through the upper and lower part. Both parts of the 
lens act as prisms, bases in the middle. 




true position of an object, we see it through the upper 
lens at a, and through the lower at b , but never where it 
really is. People who wear such glasses, may, by look¬ 
ing, while descending the stairs, through their lower 
part, reach the bottom sooner than they expected; and 
if they h&ve not lost their spectacles by the accident, 
may stare in bewilderment through the upper part at the 
place whence they came so suddenly. This has hap¬ 
pened more than once. 















71 — 


The relation of distant glasses to reading glasses is 

o o o 

calculated by the following rule in the inch measure: 

Multiply by 3 from + 16 to + 11 
“ “2 “ . + 10 to + 6 

“ “ 1J “ + 5 down. 

When people wear + 16, we try + 48 or + 60 
“ “ “ + 11, “ “ + 30 “ + 36 

“ “ “ + 10, “ “ + 20 44 + 24 

and we will find that one of these numbers is generally 
correct. In all cases where people insist on having 
double-focus glasses, we should persuade them to take 
split glasses. Some, however, think themselves smarter 
than the doctor, and will persist in using double-focus 
single glasses, because they look better. Let them have 
their way, and be happy. 


CHAPTER X. 


Colored or Tinted Glasses. 


It took the human race very long to invent suitable 
appliances for the protection of injured and diseased 
eyes. Every other want was early cared for, according 
to existing means and ability; but we look in vain for 
anything more ingenious than the simple application of 
rags or a handkerchief to an inflamed eye. We may be¬ 
lieve with certainty that men, when hardly above the 
level of their surroundings, used the skins of animals to 
protect themselves against the inclemency of the weather. 
But among the thousands of improvements they made 
with advancing civilization, there was none to benefit the 
ailing, suffering eye to any extent. This organ, so deli¬ 
cate, and yet so recklessly overtaxed, is nevertheless a 
most obedient slave, always ready to perform its task 
from morning to night, till finally, unable to stand any 
longer the effect of the glaring, piercing light, or bear 
its other hardships and abuses, it closes its shutters, and 
alas ! what becomes of its owner, so suddenly deprived 
of its services? The terms, blind men and beggars, are 
almost synonyms, and indicate the great misery attend¬ 
ing the loss of sight. Fortunately we live in an age in 
which science has also investigated this, so long neglected 
part of the “ evils that befall mankind,” and we can say 
with pride: the blind shall see , not by a mysterious won¬ 
der, but by the scientific skill of experts. 

Among the most useful modern appliances to relieve 
the sufferings of an afflicted eye are the so-called Pro¬ 
tection Spectacles , which are set with colored lenses, to 




— 73 — 

soften the excess of light, otherwise so annoying and 
hurtful. Since spectacles' were invented, people have 
made experiments with different colors, giving prefer¬ 
ence at one time to this, at another time to that color, 
according to fashion, entirely disregarding optical laws, 
till they have settled for the present, with scientific rea¬ 
sons, upon the tint of smoke. To' comprehend this ques¬ 
tion thoroughly, we must direct our attention first to the 
theory of colors in general, and see what we understand 
by the term “spectrum.” 

When we speak in an optical sense of colors, we ex¬ 
clude, of course, the pigments used by painters, who in¬ 
clude among them even black and white , which are no 
colors at all. Black is the absence of light, and conse¬ 
quently of colors; white is the undivided light, contain¬ 
ing all colors so combined that the different tints totally 
disappear. White light is, therefore, called “color¬ 
less,” although it needs only to pass through a certain 
medium to be resolved into the brightest colors, as is 
seen in the rainbow, where the falling drops act as the 
decomposing agent. The rainbow is a fair specimen of 
the Solar Spectrum , showing the seven spectral colors, 
red, orange, yellow, green, blue, indigo and violet. To 
these should be added brown , outside of the red, and 
gray , outside of the violet. By means of a prism we are 
enabled to produce this spectrum to perfection, and to 
investigate the particular properties of each color sepa¬ 
rately. We thus find that red is least refracted. It 
forces its way forward like a heavy ball or shot, while 
violet is the most refracted, yielding to the obstruction 
it encounters in passing through the prism. Scientists 
have found that the waves of red are nearly twice as 
long as those of violet, and this accounts for the impetus 
ousness of its ray, which almost overcomes the interfer¬ 
ence of the prism. The waves of the other colors be- 
8 


— 74 - 


come gradually shorter, up to violet; and as the smaller 
waves act more gently upon the tender tissues of the 
retina, we might guess with some probability that violet 
would be the softest color to the eye. This would be a 
gross error. There is a decided difference in the effects 
of colors on the eye. It is pleasant to look at the dark 
green of a meadow or the foliage of trees; but it is very 
trying to use green spectacles, because our eyes are then 
constantly under the influence of one particular color. 
In fact, no color is hurtful to the eye as an object to 
look at; but if a special color is used as the medium to 
look through, it always acts more or less injuriously, as 
the shade is lighter or darker. There is no exception to 
this rule. We must bear in mind that a healthy eye is 
able to endure the full force of the whole light, and that 
any division and exclusion of its essential components 
will act detrimentally, as would be the case in breathing 
only oxygen or nitrogen separately,when the mixture of 
both in a certain proportion is a vital condition of our 
existence. No separate color is, therefore, a proper 
substitute for white light, for which our eye is con¬ 
structed, and so w r ell adapted as long as it is in a normal 
condition. 

But when the eye is impaired, and cannot stand the 
full strength of light, should we not shut off the most 
hurtful part of the spectrum, and allow only the softer 
colors to act upon the tender organ? Does not the phy¬ 
sician regulate the diet of his patient by depriving him 
of certain food? Certainly, so it seems at first to 
any superficial observer, but even the most rigorous diet 
does not deprive the patient of any of the necessary 
elements of his nutriment; only quantity and form are 
modified. Of the fifteen elementary substances our body 
contains, the four most essential are oxygen, hydrogen, 
nitrogen and carbon. To eliminate from the diet of a 


- 75 — 


patient one of these four elements, would not be more 
irrational than to suppress any color of the spectrum in 
favor of another. Neither green, blue nor violet can be 
substituted for the peculiar union of all colors producing 
white light. Any shifting of the finely balanced ingre¬ 
dients of white light will act fatiguingly or even per¬ 
niciously upon the eye. Those of my readers who un¬ 
derstand chemical formulas will readily see the point in 
question. The (old style) formula of sugar is expressed 
by C 12 H n On. Now take two atoms, each, of hy¬ 
drogen and oxygen from the molecule, and we have 
vinegar C 12 H 9 O 9 . By the same process “ sweet” 
light may be made disagreeable by smothering one or 
more colors of the spectrum, or rather by increasing the 
effect of one particular color at the expense of the 
others. 

I have treated this subject more elaborately than 
would have been necessary, if other writers had given it 
the close investigation demanded by the importance of 
the scientific principles upon which it is based, and the 
practical service which a thorough knowledge of them 
will enable us to render those whom it is the object of 
our trade to protect or assist. 

We have seen that the exclusion of particular colors 
of the spectrum does not answer our purpose. It re¬ 
mains, therefore, to decide what can be done to protect 
the suffering eye from the injurious effect of light, with¬ 
out interfering with the essential combination of the 
thermic, electric and magnetic qualities of the sun’s 
rays, which peculiar combination agrees exactly with the 
construction of the eye, as the milk of the mother 
agrees with the constitution and healthy development 
and growth of her infant. The most rational method is 
to diminish the whole amount of light by smoked glasses. 
These do not alter the proportion of the different col- 


- 76 - 


ors, and produce no change in their vibrations. They 
only lessen the amount of light without disturbing the 
proportion of its elements. The whole spectrum is thus 
uniformly reduced, and nothing is changed by smoked 
glasses but the strength of the excessive light. 

To show that no special color by itself will satisfy the 
djye, I remind the reader here of the well-known experi¬ 
ment of saturating the eye with one color by excluding 
the others, and observing how eagerly the eye absorbs 
the complementary color after the test-color is suddenly 
removed. The easiest way to make this experiment is to 
cut from colored paper round pieces of the size of a sil¬ 
ver dollar. Lay one of these circles upon white paper, 
and look for a while steadily at it, the eyes one foot 
from the colored circle. By removing this quickly, and 
looking always at the same place, we will see distinctly 
the complementary color. If the circle was red, we will 
see instead of it a green one, which color is comple¬ 
mentary to red. A yellow circle will produce violet; 
blue produces orange; and green will show red. The 
eye seeks to be relieved from the strain, and is, there¬ 
fore, much in need of the missing colors. It takes, in¬ 
deed, a good while before the eye recovers from the fa¬ 
tigue, and is again able to receive the white light with¬ 
out seeing colors. This experiment was known for 
many years, but nobody has yet drawn that lesson from 
it which it so clearly teaches. Medical books leave the 
selection of a special color an open question, and per¬ 
mit the patient to choose for himself, or they are preju¬ 
diced in favor of one particular color, as the celebrated 
Dr. Graefe was towards blue glasses, rejecting smoked 
almost entirely. 

It is needless to waste words further in regard to 
green, blue or violet spectacles, still manufactured and 
sold extensively to persons, who are always on the look- 


- 77 — 


out for something different from what others sell, and 
which are recommended the higher, the less such “ opti¬ 
cians ” know of the science of their trade. I conclude 
this chapter by citing some sensible remarks of Arthur 
Chevalier: “ The great trouble is that the manufacture 
of colored lenses is not yet scientific. There are thou¬ 
sands of different shades, due to the careless way in 
which glass is made. If scientific glass-manufacturers 
would take it in hand to produce a clear colored crown- 
glass, and would publish their formula, after their glass 
has been approved by leading oculists, all colored leases 
could be limited to one dozen different shades, classified 
with the same certainty as we define now the white lenses 
by diopters. As colored lenses have only the object of 
softening the excessive light, it is rational to imitate the 
common practical way of shutting off the light by 
closing, according to necessity, the blinds of our win¬ 
dows or turning down our lamps.* This is done by 
smoked lenses in their different shades. There is hardly 
any exception in all the many defects of diseased eyes 
where smoke would not do all services expected from 
colored spectacles. Even healthy eyes are in need of 
them in countries covered with snow, or where the in¬ 
tense glare of a tropical sun affects them. This refers 
exclusively to people who are obliged to expose them¬ 
selves only occasionally to the disadvantages of the cli¬ 
matic severities, and not to those who are habitually 
accustomed to them. The Esquimaux make for them¬ 
selves from wood a kind of coquille spectacles with a 
slid in the middle, to allow only a limited quantity of 
light to enter the eye.” 

femoked glasses are absolutely necessary when the eye 
is inflamed, after most operations, and in other cases 
decided upon by oculists. 


CHAPTER XI. 


The Proportion of Caloric Rays in the Different 
Kinds of Light. 


According to the old emission theory, light is a com¬ 
pound matter; but, according to the new undulatory 
theory, it is a compound force. It is a mixture of lu¬ 
minous and caloric rays, and is also a combination of the 
different colors. To resolve light into its colors has been 
a comparatively easy task, especially since the proper¬ 
ties of the prism have become known; but the complete 
separation of luminous from the caloric rays is yet a 
matter of investigation. Eminent scientists have labored 
long to isolate one'from the other, but with only partial 
success. Light, passing through an ice block, or thiough 
plates of mica, is not entirely deprived of its caloric 
rays, although they are absorbed to a certain extent, 
but by means of a strong burning-glass you will detect 
enough of them to be sensible of their presence. Some 
explorers have succeeded in completely absorbing the 
luminous rays, and showing at the same time the pies- 
ence of the caloric rays in their full strength. 

The following experiment was communicated to me by 
Professor Pepper, of England, in 1872, when he, on his 
American lecture tour, passed through New Orleans. I 
repeat it here as he explained it to me. I have never 
tried this experiment myself. I remember with great 
pleasure his able lecture on ii Light and Heat, illustia- 
ted profusely by novel and highly interesting experi¬ 
ments. (See Cut on next page.) 

The candle b stands between the glass-jar c and the 




— 79 — 



concave mirror a. The rays of the candle are thrown 
by the mirror on the flat jar, filled with a solution of 
sulphuret of carbon and iodine , which completely ab¬ 
sorbs the luminous rays. You cannot detect through the 
jar the least trace of light; but if you hold your finger 
at the point d you will find that the caloric part of the 
light is concentrated there most keenly. This shows 
that the liquid absorbed only the luminous rays, and 
allowed the caloric rays to pass through without percept¬ 
ible interference. 

In the same manner that luminous rays are modified or 
intercepted while passing through bodies of different de¬ 
grees of clearness, the caloric rays are also more or less 
intercepted by different substances. Mica, for instance, 
absorbs the greater part of the caloric rays; but the only 
substance which allows all caloric rays to pass without 
any obstruction is clear rock-salt. Experiments with 
prisms of this salt have demonstrated the fact that light 
passing through such a prism gives two spectra, one by 
the luminous, another by the caloric part of the light, 
with the remarkable difference that the red line of the 













— 80 — 


caloric spectrum is as broad as all the other colors com¬ 
bined, from orange to violet. This experiment is an un¬ 
deniable proof that the caloric and luminous rays can be 
separated, and that both kinds of rays are subject to the 
same law of nature, the undulatory or wave theory. 

This theory defines light as motion of such an intense 
Velocity that we can express it only in figures, but are 
utterly unable to comprehend it. Imagine that we were 
able to build a machine of indestructible material, and 
had the power of increasing its revolutions indefinitely. 
We put it into operation. As long as we can follow its 
movements with the eye, we have common motion. We 
can follow with our eyes a stone thrown at some dis¬ 
tance. This also is common motion. Let us now in¬ 
crease the speed of our machine thirty-three revolutions 
a second. The eye can no longer follow it, but the ear 
discerns a low hum, which becomes louder as the ma¬ 
chine gradually moves quicker. We have sound. A 
rifle-ball is not seen, but we hear its whistling noise. 
When the tone has reached its highest pitch (38,000 vi¬ 
brations in a second), our ear is unable to perceive any 
further increase; Ave feel then the effect of heat , and 
soon see a violet glimmer, then a transition through 
blue, yellow and red into white. We now have light. 
The vibrations have increased to many thousand billions 
a second. If our machine is not melted by this time, 
and is still running with increasing speed, we had better 
keep at a safe distance, for the next action will be the 
emission of electric sparks and lightning in all direc¬ 
tions. 

Here science ends; and here is the limit of all power 
and force we can explain or comprehend. But if we 
allow our imagination its widest range, and look upon 
this experiment only as the symbol of the universal, sub¬ 
lime power, does it not give us a faint idea of the proper 


— 81 — 


mode of attaining to the knowledge of this ultima ratio, 
the incomprehensible omnipotence? 

We have seen that light and heat are always combined, 
that there is no light without heat, for phosphorescence 
cannot be regarded as light. There is natural light as 
the sun, and artificial light. In ancient times people used 
torches or splinters of pine wood to light their rooms; 
these enabled them to distinguish objects but faintly. 
Oil-lamps and candles were a great improvement, and 
by their aid people could continue their work after sun¬ 
set. Gas was a still greater improvement; but the 
greatest of all is undoubtedly the electric light. I think 
that with this we have reached the climax of lights for 
many years to come. But what percentage of caloric 
rays is contained in these different lights? Every one 
has observed that in a well-lighted theatre or ball-room 
the light is quite dazzling, although it cannot be com¬ 
pared to daylight. It is, therefore, not the excess of 
light which fatigues the eye, but the excess of the calo¬ 
ric rays which makes it so unpleasant. The cause of this 
is easily explained. A small quantity of light dilates 
the pupil; the heated rays enter more freely, and soon 
cause the eye to smart and ache. 

Sunlight is the coolest; it has only 70 °/ 0 of caloric, 
and 30 °/„ of luminous rays. It is perfectly white or 
colorless, and is the most agreeable to the eye. The 
caloric part of it is greatly modified by the moist atmos¬ 
phere it has to penetrate, and by repeated reflections. 
The emmetropic eye is, therefore, well able to bear its 
effect the whole day long without fatigue. 

The Electric light comes next. It is slightly colored, 
has a violet-bluish tinge, and contains 80 % of caloric 
rays. If this light is steady, and the eye is shaded 
9 


— 82 — 


against the entrance of the direct rays, it will rank next 
© 

to the sunlight. 

Then comes the ordinary Oil-lamp, including the im¬ 
proved coal-oil lamps, which have 87 °/„ of heat rays. 

Gras, with its 90 °/„ of caloric rays, is the best agent 
opticians could employ to increase the sale of spectacles. 
The light is yellow, as we readily see when a gaslight is 
near an electric light; besides, it is very often flickering 
and unsteady. For home use, a good coal oil lamp is 
preferable under all circumstances. 

The poorest light of all is the Alcohol lamp, which has 
only J °/ 0 of luminous rays, and is absolutely unfit fox 
seeing purposes. 

In recapitulating the foregoing by comparing artificial 
lights with sunlight, we find first that they are always 
colored; and secondly, that they contain an excess of 
heated rays, i. e., there are less luminous and more ca¬ 
loric rays in them than in the colorless and compara¬ 
tively cool sunlight. And as we know that heat is the 
principal cause of weariness and fatigue of our eyes, 
we have here a guide in selecting for working purposes 
just that light which will tax them the least, and will not 
spoil them prematurely. 


CHAPTER XII. 


Second Sight, Itg Cause and Final Course. 


Occasionally we see old persons rejoicing in a re¬ 
newed growth of hair after having been bald-headed for 
years. Their natural hair again appears as it did in 
their youth; not white and bleached, like the last rem¬ 
nants, but blonde or dark colored, and even curled, a 
luxury they never dreamt of in younger years. And it 
is not only the hair that manifests a certain kind of re¬ 
juvenescence, the toothless jaws produce another set of 
teeth, and what is still more wonderful, the eyes regain 
their youthful strength, and these “ old lads lay aside 
their spectacles and read once more without them as 
they did in their younger days. The reproduction of 
hair and teeth is generally little noticed by the skeptical 
public, as these changes can be produced by wig-makers 
and dentists; but the reading without spectacles at that 
age, by those whom people have seen using them so 
long, cannot be the result of any deception, and that 
there has been a real change in their eyes, is a fact that 
cannot be denied. 

This freak of nature is not the renovation of the 
worn-out machinery, as many people think. It is the 
last desperate rise of an extinguishing light. Let us 
look into the cause of it. The lens of a normal eye has 
a certain convexity which in the length of time flattens, 
and compels people to remedy this loss by convex spec¬ 
tacles. The more the crystalline lens flattens, the 
greater must be the convexity of the spectacle-glasses. 
One necessitates the other, up to old age. But it hap- 




— 84 — 


pens now and then that the lens no longer flattens. On 
the contrary, it is contracted and rounded up again by a 
degeneration of its tissues, causing the phenomenon of 
.the so-called “ Second Sight.” Such people have to take, 
gradually, weaker spectacles, till they are able at last to 
read without them, and are even benefited by concave 
glasses for distant vision. If this changing process of 
the eye would stop here, and would last many years in¬ 
stead of a few months, it would be well; but this appar¬ 
ent improvement of the eyesight is only the manifesta¬ 
tion of a serious disease of the crystalline lens, which is 
undergoing an alteration similar to that of an orange, 
when its juice dries up, and it gets hard and wrinkled. 
So it is with this delusive present, which unveils once 
more the faint image of our youth, only to fade away 
the more rapidly from our eager grasps, in less time than 
it took to develop itself. The humor of the crystalline 
lens loses its transparency, the pupil, which was for¬ 
merly jet-black, appears grayish, and cataract has fairly 
commenced to destroy sight for ever. Is it not a 
wretched delusion which makes people rejoice for a while, 
and then ends in the misery of total blindness ? 

I advise my readers, who have to deal with such cus¬ 
tomers, seeing the advance of this fearful visitation, not 
to be indiscreet, and wantonly dispel their happy decep¬ 
tion, as nothing in the world can arrest the final course 
of their trouble. You may advise them not to read or 
sew at night, and to spare their eyes as much as possi¬ 
ble. When in the first stage of incipient cataract, bright 
light begins to annoy their sight, give them smoked 
glasses; these neutralize best the scattered rays, which 
pass through the infected lens. Do not lose patience by 
their renewed attempts to find some relief by changing 
their spectacles. Have always a kind word for them, 
and as you cannot help them materially, let them have 
the full benefit of your benevolent sympathy. 


CHAPTER XIII. 


History of the Invention and Introduction of 
Spectacles. 


Old tradition credits Phoenician merchants with the in¬ 
vention of glass. This nation occupied a part of the 
coast of Syria, between the Lebanon and the Mediterra¬ 
nean sea, northwest of Palestine, and w T as already widely 
known at the time of Jacob, the patriarch, about 1750 
years before Christ. But it seems glass was known be¬ 
fore that time, as there has been lately found below the 
ruins of old Nineveh a lens evidently used for optical 
purposes. The knowledge of the manufacture of glass 
was early acquired by the Egyptians, who improved on 
it, and made even colored specimens. After the Romans 
conquered Egypt, this art was introduced into Italy, 
where they soon learned to make plate-glass, and also 
produced a kind of glass which could stand without in¬ 
jury the effect of hot fluids. They also claimed to have 
known a glass which was malleable, and to a certain de¬ 
gree unbreakable. A good story in relation to this 
states that a man once demanded an audience from the 
Emperor, and presented to him a goblet of glass. The 
Emperor was highly pleased with the splendid workman¬ 
ship of it, but when it passed from hand to hand among 
the courtiers present, it accidentally fell to the floor, or, 
as it is also related, the artist himself threw it willfully 
down. It did not break, but was badly dented. The 
man repaired it immediately with a small hammer he had 




brought along with him. It is a pity that this important 
invention is entirely lost. One Roman historian reports 
that Nero could not see very well, and that he made 
use of a large jewel in the shape of a lens, to enjoy a 
better sight of the lights of his gladiators. But this 
was not imitated by others, and it is narrated by the his¬ 
torian only as one of the many strange extravagancies of 
this most remarkable man of the Roman empire. * 

For the next one thousand years no advance in science 
was made. Many old inventions were forgotten for 
awhile; some are lost forever, and we only hope that 
somebody will be fortunate enough in future to redis¬ 
cover by chance or by the aid of advanced science what 
an accident once so luckily revealed to our ancestors, 
who, however, did not appreciate it, but buried their 
pound like misers. The investigating mind was for this 
long period turned into the opposite course of training. 
Instead of studying the hidden forces of nature, and en¬ 
joying its bountiful gifts which an exceedingly friendly 
providence had put into an easy reach of their grasp, 


* Nero was, perhaps, hypermetropic, but not myopic, as it is often stated. 
Myopia was not known in olden times, because it did not exist. Travellers 
have never found among uncivilized nations a case of myopia. People who do 
not read much, or do not use their eyes for seeing small objects, are not near¬ 
sighted. In America, which is mostly an agricultural country, there are on an 
average twenty-five hypermetropics to one myopic (cities excepted), while in 
Germany, where printing Avas invented, there are twenty-five myopics to one 
hypermetropic person. Myopia is hereditary, but decreases in a few genera¬ 
tions when the cause for it is removed. It is, therefore, simply a temporary ab¬ 
normity. Either the visual axis of the eye is too long (axial myopia), or the 
convexity of the lens is increased (refractive myopia) ; it is always acquired by 
habit. We could call it a habitual anomaly of the eye ; it is of modern date, and 
was not knoAvn in ancient times. 

The causes of hypermetropia are not at all dependent upon the abuses of the 
eyes by reading or doing fine Avork. Many natural causes produce this abnormal 
condition, which is certainly as old as the human race, although it has been 
really understood and explained only in the present century. The close resem¬ 
blance of myopic and hypermetropic eyes, compelling both of them to use 
spectacles for seeing near and far, has wrongly caused many writers to make 
Nero near-sighted. 



- 87 - 


people turned their eyes to the clouds, till they lost 
sight of their beautiful surroundings. Unfruitful dog¬ 
mas, deceptive sophistry and blind hatred were substi¬ 
tuted for the great civilizer Science. 

The first information of the use of a magnifying lens 
we find in a book about optics by Alhazen, an Arabian, 
some 900 years ago. Science at that time was like a 
smothered fire, with only now and then a spark faintly 
glowing in the ashes. Then there is another lull in the 
development of our interesting subject, till Roger Bacon 
made it again a special study, and even succeeded in 
grinding magnifying lenses himself. He was a professor 
at Oxford, England, and being ahead of his time in sci¬ 
ence, was misapprehended and persecuted by his con¬ 
temporaries, as happened to Galileo and other men of 
genius. Bacon died in 1294, and it is not stated that he 
combined the magnifying lenses into spectacles, which 
we are told was accomplished about that time in Italy. 
We may point out the year 1300 as the one wherein 
spectacles were invented , if we ignore the pretentions of 
the Chinese, who claim to have known them long before 
that time. However, this may be, all inventions made by 
them were barren to the rest of mankind in consequence 
of their exclusiveness. 

An old Latin document found at the Convent of St. 
Catharine of Pisa, of the year 1303, tells us that a 
Monk, Alexander of Spina, who died in 1313, was so 
skillful a mechanic that he could reproduce any kind of 
work he had seen, or which had been described to him: 
and that he made spectacles after having seen them, and 
the inventor had refused to communicate the true pro¬ 
cess of. their manufacture. This selfish inventor was 
probably Salvino Armato, on whose tombstone was the 
inscription: 


— 88 — 


Qui Giace 

Salvino D’Armati Degli Armato 
di Firenze, 

Inventore Degli Occhiali, MCCCXVII. 
Here rests 

Salvino, etc., Armato 
of Florence, 

Inventor of Spectacles, 1317. 


The use of spectacles spread very slowly, because 
people had little need of them. Only a limited number 
of men could read, books were very scarce and very 
dear. Printing was not yet invented, all books were 
written by hand, and it was only afterwards, when their 
circulation increased, that spectacles came into demand. 
An old chronicle of Nuremberg, in Germany, of the 
year 1482, mentions that there were some manufacturers 
of spectacles in that city. 

Spectacles were for a long while merely an object of 
curiosity, and were made use of as*a novelty to improve 
appearance, as some years ago every “ Dude, 5 ’ male or 
female, had to wear bliie glasses for fashion’s sake. In 
Spain they formed a part of the costume of every well- 
bred person. This absurd use of glasses was meant to 
increase the gravity of the appearance, and conse¬ 
quently the veneration with which the wearer of them 
was regarded. The glasses were proportional in size to 
the rank of the wearer. Those worn by the Spanish 
nobles were sometimes three inches in diameter. The 
Marquis of Astorga, when having his bust sculptured in 
marble, particularly enjoined upon the artist not to for¬ 
get his beautiful spectacles. 

After this first foolish introduction of spectacles, 



— 89 


which was quite contrary to their real mission of being 
an indispensable companion of old age and a general 
benefactor, there was again a period of about three 
hundred years in which little or nothing was said about 
them till some scientists, especially astronomers, experi¬ 
mented with the different kinds of lenses, and succeeded 
at last in constructing telescopes. The necessarily correct 
finish of such lenses gave a new impulse to the manu¬ 
facture of spectacles, although they were made yet in 
limited quantity by solitary workmen, and by hand. 
These congratulated themselves when they could occa¬ 
sionally sell one pair. Spinoza , who died in 1677, had 
also learned the art of glass-grinding to make his scanty 
living, while he was writing his philosophical works. It 
is related that he made a pair of spectacles for the cele¬ 
brated German philosopher Leibnitz, who formed his 
acquaintance at Haag (Holland). 

From the hitherto sole occupation of opticians as 
glass-grinders and manufacturers of spectacle frames, 
they had soon to extend their vocation in proportion to 
the demands of science. They had not only to make 
spectacles and magnifying lenses, but also spy-glasses, 
microscopes and telescopes, and had to combine, of 
course, their trade with that of a regular mechanic. At 
last the medical fraternity came forward with their share 
of optical work, and it was now time to divide the trade 
into different branches. Some devoted their whole at¬ 
tention to the manufacture of astronomical instruments; 
others to mathematical and philosophical; others to 
optical appliances, and all under the collective denomi¬ 
nation “ Opticians,” with the same right that Surgeons, 
Oculists, Pathologists and Dentists claim to call them¬ 
selves “ Doctors.” By this division of work the public 
in general must profit, if every one strives to be per¬ 
fect in his individual part. 

10 


— 90 ■** 


It is only within the last century that our trade has 
risen to that great prominence it occupies to-day. We 
are now an indispensable factor in all scientific pursuits, 
and furnish instruments, not only the most scientific, 
but also the most useful ever offered to benefit the 
world. We have reason to be proud of our achieve¬ 
ment, but we must not forget that we were merely the 
tools, executing the orders of scientists, who did the 
brain work for us, as Newton, Brewster,. Hersbhel, 
Euler, Wollaston, Donders, Helmholtz, Graefe, Kirch- 
hoff and many others, and that we have not many opti¬ 
cians like Frauenhofer to boast of. He, for instance, 
was a practical workman and a scientist at the same 
time. He gave the first impulse to Spectrum Analysis, 
and also manufactured, flint-glass of such perfection as it 
has been impossible to produce since his death (1826). 
But alas ! he was also one of those narrow-minded work¬ 
men who write on their workshop “No Admittance . 99 

Flint-glass was known over three hundred years ago. 
There was as early as 1557 a factory of it in London, 
and English flint-glass was considered the best in the 
market. But the}^ never could make pieces of more than 
a few inches in diameter, suitable for astronomical pur¬ 
poses, till Frauenhofer astonished the world with a lens 
of almost a foot in diameter, which was set afterwards 
into a refractor for the observatory at Dorpat, in Rus¬ 
sia, and is yet in use. The difficulty is that the great 
quantity of lead in flint-glass cannot be equally distrib¬ 
uted throughout all parts of the lens. Frauenhofer took 
his secret with him into the grave. 

The spectacle business advanced considerably after the 
oculists detected the asymmetrical refraction of the cor¬ 
nea, called Astigmatism. Thos. Young, of England, 
made the first studies in astigmatism in 1793, but it was 
little noticed by his contemporaries. It was only after 


91 - 


Donders, Javal, Knapp, Helmholtz and others, more 
than fifty years afterwards investigated it, and explained 
the method of its correction by means of cylindrical 
lenses , that it was generally understood. The manufac¬ 
ture of such cyl. lenses with all their combinations, and 
especially their correct setting , was a new departure in 
our trade, and many opticians were considerably troubled 
before they fully mastered the difficulties in connection 
with this most delicate correcting medium in the shape of 
spectacles. A competent optician of I860, falling 
asleep like Hip Van Winkle , and awaking to-day, could 
not fill the simplest order of an oculist, but would have 

to learn his trade over again. 

© 

As long as the selection of spectacles was left to the 
opticians, they contented themselves with the correction 
of a limited number of defects, and declared the balance 
incurable. They did not know the nature of irregulari¬ 
ties, such as Hypermetropia, Astigmatism, etc., and 
were, therefore, totally in the dark about their correc¬ 
tion. Oculists formerly considered it beneath their dig¬ 
nity to have to do with spectacles, and after they had re¬ 
stored the injured or suffering eye to a healthy state, 
they turned the patient over to an optician for the proper 
selection of glasses, unconcerned whether this selection 
was a good or bad one. It is only since Donders, Helm¬ 
holtz, Graefe and others investigated such “ incurable ” 
cases, that they are understood, and can be thoroughly 
corrected by spectacles specially adapted. Although 
such spectacles are manufactured by opticians, the 
credit of their beneficial action belongs to those eminent 
explorers who gradually wrenched their selection from 
the hands of mostly indifferent mechanics, who destitute 
of the necessary scientific education, have to satisfy 
themselves with a secondary position under the leader¬ 
ship of the oculists. There is no blame attached to our 


— 92 - 


present position, as it is not at all a step backward. On 
the contrary, the standard of our trade has advanced 
considerably, but it has not kept step with the gigantic 
progress of Ophthalmology, which has no equal in medi¬ 
cal history. In the last twenty-live years Ophthalmology 
and general Surgery have become exact sciences, while 
the rest of medicine is yet for the most part empirical, 
as was the case with our mechanical and hap-hazard man¬ 
ner of selecting spectacles, when the patient was the 
principal judge of their correctness. 

The selection of spectacles in complicated cases is now 
the exclusive dominion of oculists, who are, as physi¬ 
cians, qualified to prepare the eye for a thorough exami¬ 
nation. Any optician, tampering with the eyes of an 
easily frightened customer, may cause himself great 
trouble if he cannot legally attach to his name an M. D. 
Only cases of simple presbyopia, manifest myopia and 
hypermetropia can be properly investigated by an opti¬ 
cian, because the other and more complicated errors of 
refraction require that the ciliary muscle (accommoda¬ 
tion) be temporarily paralyzed by a mydriatic, as atro- 
pia; and that in this state of the eye accurate and re¬ 
peated measurements be made with test-types and trial 
lenses. Signs in the windows of opticians which read: 
“ Examinations of the eye made free of charge,” smack 
of quackery, and should be removed.—We have here to 
record also the meritorious invention of Jaeger and 
Snellen, whose test-types are so universally in use at 
present. 

Among all the men I have mentioned, there is no 
American; and we must concede that as to the theory of 
our trade, Europe is ahead of us. But as to the practi¬ 
cal part, we have in the short space of the last twenty- 
five years outdone all former efforts of Europe. Before 
that time most spectacles were imported from France, 


93 — 


England and Germany; but lately we only import the 
lenses, and I may predict that in less than another quarter 
century this importation will have ceased, as well as the 
former brisk trade in European watches , which has been 
stopped entirely by the superiority of our own manufac¬ 
ture. We may be proud of our well-earned success, and 
I am not astonished when our people return from Eu¬ 
rope, telling the gratifying news that their glasses with 
the latest improvements, were admired by every European 
optician who had seen them. 

In concluding this chapter, let us devote a few lines to 
the origin of the different names under which spectacles 
are known. The English word “ Spectacles ” is derived 
from the Latin word spectare , to look at, to behold, and 
is used always in the plural on account of the two glasses 
of which they consist. The French word “ Lunettes ” 
comes from the word lune , moon; and lunettes means 
little moons. Spectacles were first made round, and 
although they were changed now and then according to 
fashion, the round shape survived all other patterns 
down to recent date, since which it has been superseded 
by the oval shape. The German word “ Brille ” is de¬ 
rived from Beryl , a transparent greenish-bluish mineral, 
called by the jewelers Aqua Marine. The Latin name 
for it is Beryllus, and signifies to shine or sparkle. The 
word Brilliant is also derived from Brille, but is now 
used only in reference to diamonds. In former years 
people in Germany called all colored glasses ‘-Berylle,” 
and as a great many spectacles, especially those worn for 
fashion’s sake, were set with plain colored glasses, this 
optical instrument received its name from that mineral. 


MISCELLANEOUS APPENDICES. 


A. _How to Believe An Injured Eye. 

This article is not altogether my own, but is partly 
compiled from different sources. It would not have 
found a place here if it were not for the great useful¬ 
ness of these simple directions in case of emergency. 
Though the eyes are well protected and shielded by the 
forehead, the nose-bridge and the cheek-bones, they are 
nevertheless much exposed to accidents caused by flying 
objects; and although the eyelids are the most reliable 
safeguards to keep off any foreign intruders, they may, 
be out-generaled occasionally, when they are the least 
aware of any danger. Some injuries do not allow of any 
delay, and as medical assistance is not always to be had 
when mostly needed, I thought it proper to add this 
treatise not only for the personal benefit of my readers, 
but also for that of their friends and customers, who 
may in their trouble come running to the optician as the 
next proper person to give them relief . I was several 
times successful in this regard, and may say that I saved 
more than one eye from great annoyance and danger. 

A very common accident is the flying of mud , dust or 
insects into the eye, which, by the closing of the eye, 
enter between the lid and the eye-ball. People thus 
affected generally keep their eyes closed, as the opening 
of the lids causes such an irritation that the eye-ball is 
soon inflamed, and turns red and bloodshot. The quick¬ 
est way to relieve those sufferers is to wash the dirt out 


— 95 — 


with clean water bv means of a camel-hair brush or a 
feather. This is done in the following manner: With 
oui left hand we take hold of the eye-lashes of the upper 
lid, drawing it forward enough to allow the brush or 
feather, previously dipped in water, to enter between the 
eye-ball and the lid, till we reach the inner folds. We 
direct the patient to look downward, and move the brush 
towards the nose, not to the outside. We have to repeat 
this several times with plenty of water. Then we de¬ 
press the lower lid, directing the patient to look up¬ 
wards, and wash carefully as before, cleaning the brush 
after each application. In some trilling cases, when an 
insect or a few grains of dust have entered the eye, it is 
sufficient to draw the upper lid as far down as possible, 
and a little outward, and push the lower ome as far up as 
you can. Then let the upper lid fly back to its natural 
position, when the eye-lashes of the lower one will act as 
a brush, detaching any light substances, and relieving 
the eye instantly. Make it a rule never to rub the eye 
when injured, as the irritation will be increased largely 
by it, and soon will cause inflammation. When hard 
pieces are imbedded in the tender parts of the con¬ 
junctiva, which cannot be removed by the brush, it is 
not difficult to remove them if they are lodged in the 
lower lid, by means of a handkerchief or some small 
pincers; but it requires some skill to remove them from 
behind the upper lid. In order to accomplish this, we 
have to evert the same, which is done by taking a good 
hold of the eye-lashes and the edge of the lid with the 
left hand, and applying with the right hand a thin pencil 
or any other rounded object to the middle of the lid, and 
by depressing the pencil, and at the same time swinging 
the left hand upward, the lid is everted and the inside 
exposed for examination. The patient is now directed 
to look downward, which brings into view the whole 


— 96 ~ 


inner surface of the upper lid, and enables us to remove 
any foreign bodies, as grains of sand or bits of coal, yet 
sticking in the soft part of the tender tissues. 

Mechanics are very often hurt by flying particles of 
metal while hammering or turning, and chips may strike 
and penetrate to some extent the front part of the eye. 
If these are of iron or steel, and not imbedded too deep, 
we may remove them by the use of a strong magnet. In 
case these chips have penetrated so far that the con¬ 
junctiva has closed over the entrance of the wound, it is 
necessary to consult a physician. Such wounds are not 
very painful at first, and the application of water or oil 
may be sufficient to allow us to wait even until the next- 
day to look for relief. Any longer delay may prove 
fatal, as a neglect will surely result in a violent inflam¬ 
mation, if these particles are not removed in due time. 

Another danger to the eyes is the splashing of quick¬ 
lime into them, causing sometimes the complete loss of 
sight. I myself was a victim of such an accident at the 
age of four years. Some workmen were slacking lime, 
and I was wondering how stones covered with water 
could boil. Wholly absorbed by this phenomenon, one 
mischievous boy gave me a push, and I fell headlong 
into the hot lime water, but was immediately rescued, 
washed and brought to bed. I soon felt that something 
soothing was applied to my eyes, which relieved them of 
the burning sensation. It was three weeks before I 
could open my eyes again, and I remember very well the 
many anxious inquiries of my parents, whether I could 
see them. In such accidents, the lime should be in- 
stantly washed with water and vinegar as thoroughly as 
possible, and a rag saturated with sweet-oil applied, till 
a physician can be consulted. 

If corrosive pigments and acids enter the eye, the 
whole face, eyes open, should be repeatedly dipped into 


— 97 — 


water in order to dilate and wash off the acid or paint; 
then apply freely milk, and afterwards plenty oil, till 
medical assistance can be procured. Whatever is done 
must be done quickly, as it is of the greatest importance 
to relieve the eye instantly from the ravages of such 
corrosive substances. 

In case the eye should be scalded or injured by the 
spattering of hot fluids, do not apply water, but only 
oil or milk, and shut off light and air by a compress of 
soft linen, thoroughly wet with sweet-oil, till the doctor 
comes. 

These directions are not intended to do away with the 
services of the physician. On the contrary, they are in¬ 
tended only to prevent as much as possible the pernicious 
consequences and further progress of such accidents, till 
professional aid can be procured. Sometimes five min¬ 
utes’ delay is sufficient to destroy eyesight forever, when 
by the prompt application of water, vinegar, milk or oil, 
the effects of such injuries would be diminished, and 
oftentimes removed entirely. 


B. — How Fau Can We See? 


When any-one draws the attention of his companions 
to a distant object, there is likely to be at least one 
among them unable to perceive anything, although the 
others see it well. He may be near-sighted, or have 
some other defects of vision that will prevent his seeing 
at a distance. But those who saw the object clearly on 
that day are perhaps unable to discover it on another, 
when the atmosphere is cloudy. This is the reason why 
it is almost impossible to decide the above question with 
mathematical precision. In undertaking to give some 
11 




valuable information about this apparently perplexing 
subject, I presuppose that the eye is emmetropic (per¬ 
fect) ; that the atmosphere is clear ; and that the object 
we endeavor to discover is of a color readily distin¬ 
guished from its surroundings. These three conditions 
fulfilled , we can see an object which is not farther away 
than 5000 times its diameter. If a stick is 10' long but 
V' thick, its diameter is only 1", and it can be seen at a 
distance of 5000 X 1" = 417'. The length is not con¬ 
sidered in the calculation. A wall may be 1 mile long, 
but only 4' high; then 4' would be the diameter. We 
could see it on a clear day at a distance of almost 4 
miles. Distant vision is mostly favored by the summits 
of moderate mountains. The atmosphere is clearer there 
than on the plains, and vision is not obstructed by the 
unevenness of the ground.—This rule can be applied to 
some practical use. A clock-maker, for instance, re¬ 
ceives an order for a tower clock with a dial large enough 
to render the numbers visible at a distance of 3000'; 
how large must the dial be ? To distinguish the hours at 
that distance, we must be able to see the hands. How 
broad ought they to be? One foot can be seen 5000' 
away. We find by calculation that their breadth must 
be 7", at least at one point; (our eye will readily form 
the connection of this point with the centre); and that 
the length of the numbers must be 15". The diameter 
of the dial is generally 6 times the length of the num¬ 
bers. It should, therefore, be 7\ feet wide. 

According to this rule, we can find, approximately, 
the distance of any object, if we know its size; or its 
size, if we know its distance. The breadth of a man is 
1£'. If we can barely see him, he is 1£ x 5000' away, 
or almost a mile and a half. If he is dressed in white, 
and the surroundings are dark, the distance may be set 
at If miles; if dressed in black, it may be only 1 mile. 


99 - 


Here it is again seen that the rule must be modified ac- 
cording to the contrast of the colors of the objects. 

If the back ground is dark, the impression of the dif¬ 
ferent colors upon our eye range in the following order: 
White, yellow, orange, red, green, blue, violet and 
black, i. e., black disappears first, then violet, etc. 

hite on black makes the strongest impression, and is 
seen the farthest. Upon a light-colored back ground 
the effect is the reverse, with the exception of violet, 
which disappears before red. 

It is a well-known fact that all animals of prey bear 
the color of their hiding places. This enables them to 
surprise their booty without being seen from any dis¬ 
tance. The striped tiger in the Indian swamps or jun¬ 
gles resembles the environs so perfectly that his victim 
is not aware of his presence till it is too late. The yel¬ 
low stems of the reeds, and the darker ground, produce 
a striking resemblance to the skin of this voracious 
beast. This curious play of nature is called ‘‘mimi¬ 
cry,” and benefits not only those beasts, but also many 
animals which are preyed upon. 

The hunter is thus sorely vexed, and often cannot 
make use of the above rule. But in military life there 
are many occasions where it is of an immense import¬ 
ance, by furnishing an estimate of the number of the 
advancing foe, and giving time to prepare for their re¬ 
ception. 

There arises another question analogous to the previ¬ 
ous one. I refer to the fact that it is so very difficult to 
judge with any certainty the number of people congre¬ 
gated in large assemblages. 

It requires only little practice to judge also about such 
numbers of a vast throng or concourse of people in a 
street row, in a tumult, or at a public meeting. But the 
many extravagant and contradictory reports after such 


- 100 — 


an event are a good proof of the incorrectness of the 
judgment generally exercised in this regard. The easiest 
way for this kind of calculation is to measure the 
ground in square feet, and divide the number by 4, as 4 
square feet is ample room for a standing person. We 
can measure a space by walking over it and counting the 
steps. A full step (not a stride), measures on an aver¬ 
age 2£'. Suppose, at a public meeting well attended, 
the bulk of the crowd extends in one direction 60 paces 
(150'); in another, 30 (75'); we have then 150 X 75 
= 11,250 square feet, divided by 4, gives 2812; and 
with the stray people counted in, we may estimate that 
about 3000 persons were attending the meeting. The 
next day we will read in the different papers that the at¬ 
tendance was immense, and that there were at least 5000 
persons present. Others will exaggerate the number 
even from 8000 to 10,000. These mistakes would not 
happen if we only made use of the rules laid down in 
this short treatise. 


C . _Why Do We Shed Teaks When We Weep? 


The eyes of ail vertebrae, with the exception of 
fishes, and those amphibious animals that live in water, 
are provided with tear-glands, to moisten the surface of 
the eyes and the inner side of the lids. If the tears 
were stopped, the outside of the eye-ball would become 
dry and opaque, and sight be lost. As long as the tears 
flow, they are drained through the tear-duct into the 
nose, and here mostly evaporate without any further 
annoyance. But if in consequence of catarrh or othei 
cause, these tear-ducts are closed, the eyes fill with 
water which runs down the cheeks in the form of tears. 
This occurs in the eyes of men as well as of animals, 




— 101 — 


$ 

but we cannot call it 4 4 weeping; ” it is only an overflow¬ 
ing of the fluid from local and physical causes. 

No animal weeps. Real weeping presupposes mental 
emotion, based on self-consciousness. Oqly human 
beings can reflect upon their own existence, and contem¬ 
plate themselves in an objective way. Without this 
great superiority over the animals we would be unable 
to touch that responsive chord of our spiritual existence 
which makes us weep for joy, grief or pain. 

The irritation for weeping originates in the brain, and 
is conducted from there by a separate nerve to the tear- 
glands, causing a profuse secretion of moisture. For 
instance, some impudent individual wounds the tender 
sensibility of our feelings, and unable to chastise him on 
account of his rank, power or position, we are overcome 
by our impotence and mortification—rage fills our eyes 
with tears. A woman weeps sooner than a man; her 
feelings are more easily hurt; she belongs to the weaker 
sex, and in her desolate, fainting mood, sometimes over 
imaginary troubles and grievances, she weeps heart- 
brokenly. Children and silly persons very often cry be¬ 
cause they set great value on trifling objects, whose re¬ 
fusal makes them extremely unhappy. 

But weeping is not always a sign of weakness, or an 
object to be ridiculed. The greatest men on earth had 
moments of mental agitation which made them weep; 
and while listening with awe to the story of their afflic¬ 
tion, we unconsciously reach for our handkerchiefs to dry 
our-eyes. We are overcome by a certain feeling, which 
is another prerogative of the human race—sympathy. 
The power of weeping is frequently a great blessing; it 
calms and cools our over-heated brain, and may prevent 
even serious incidents. As the opening of a valve saves 
the boiler from explosion, so tears gradually melt away 


that rock which rests upon our breast, and threatens to 
choke us by its insupportable weight. 


D . —Different Expressions of Faces, Based Upon 
the Position of the Eyes. 


The common saying, that “ A good face is the best 
recommendation in life,” is not based solely upon the 
good looks or the fine appearance of a person, for this 
has little to do with the favorable impression we make, 
at least upon our own sex. It has a deeper foundation 
for its truth, which is not generally known. How often 
do we meet persons with plain faces, and even irregular 
features, and yet are fascinated by them, although their 
discourse is not marked by brilliancy of eloquence or 
depth of knowledge. We have been captured by them 
without knowing how? It is the witchery of an expres¬ 
sive eye that has conquered us; and this is the only ex¬ 
planation of the spell exercised by such persons upon 
their surroundings. 

A close observer of the facial expressions of different 
individuals will find the greatest variety in their delinea¬ 
tion, based upon the direction of the axis of vision. In 
children this axis is almost constantly parallel, producing 
the impression of thoughtlessness, or of the childish in¬ 
nocent look. With increasing intelligence the eyes lose 
their parallelism by being fixed upon objects of investi¬ 
gation. All the affections of the mind are now mani¬ 
fested by certain motions and positions of the eyes, 
which become more convergent. The lurking look of 
the convict on trial; the watchful scrutiny of the over- 
suspicious; the piercing glance of anger; the lustful 
look of the libertine; the rude gaze of the ruffian, and 




- 103 - 


the fearful glare of the maniac, all are modifications of 
the same act, produced by an increased convergency of 
the axis of the eyes. 

The gentle and refined affections of the mind restore 
to a certain degree the parallelism of the axis. It is this 
that appeals in the eye of the trusting; sparkles in the 
eye of the happy and the gay; subdues in the look of 
the affectionate and the loving; awes and elevates in up¬ 
ward gaze of piety and religion, or composes in the gen¬ 
tle regard of the devout and resigned. 

The eyes of a frightened person even diverge; the 
wish to be far away from the place of danger causes 
this spreading of the pupils and opening of the eyelids. 

In old age the axis of vision again becomes parallel. 
The passions of former years are calmed, and the mind, 
in a contemplative mood, is now diverted upon its future 
distant home. At last the eye dies in the absolute 
parallelism of the axis of vision. 


E . —Refraction and Dispersion of Light. 


Light always travels in straight lines; it is inter¬ 
rupted in its direction only by entering a medium of 
different density from that through which it previously 
moved. This change of the rays of light from their di¬ 
rect course is called the refraction of light. Glass, 
water and all transparent bodies and fluids have this 
power of refracting the rays of light, but the relation of 
the angle of refraction to the angle of incidence varies 
with the nature of the different media, each of which 
has a distinct power. The ratio or proportion between 
them is called the index of refraction. For different 
media, it is as follows: 




— 104 — 


Air.1.000 

Water.1.336 

Oil of Turpentine.1.475 

Crown-Glass.1.538 

Rock-Crystal.1.548 

Flint-Glass..1.633 

Strass or Paste.2.028 

Diamond.2.439 


In this table, air is taken as the unit of comparison. 
The refractive power of crown-glass and pebbles is 
almost the same; flint-glass shows a considerable in¬ 
crease, strass more so. This is also a flint-glass, with a 
larger proportion of lead, and is known as the “extra 
white.” The high refractive power in diamonds causes 
that sparkling clearness called “ first water,” and is val¬ 
ued by all connoisseurs of precious stones. Spectacle- 
glasses made of diamonds would be injurious to the eyes 
on account of this glaring refractive power. 

The refraction of the rays of light passing from one 
medium to another causes also the separation of light 
into its different colored rays. This is called the disper¬ 
sion of light. We have seen that refraction refers to 
the change in the direction of the rays, while dispersion 
relates only to colors, produced by an unequal bending 
of the rays of light. The dispersive power also varies 
in different bodies; air is, of course, excluded, as we 
have no means of finding the index of its dispersion, if 


there is any. 

Rock-Crystal.0.026 

Water.0.035 

Crown-Glass. 0.037 

Oil of Turpentine.0.042 

Flint-Glass..0.049 

Diamond.0.056 
















— 105 — 

This table of the index of dispersion shows clearly 
the superiority of pebbles over any glass, because the eye 
is most benefitted by lenses of the lowest power of dis¬ 
persion. If spectacles were set with achromatic lenses, 
like the objectives of spy-glasses and opera-glasses, 
they would be the best ever made; but nobody could 
carry such a weight on his nose: besides, the high price 
of such lenses would permit only a limited sale, and 
therefore no optician could keep an assortment of them 
in stock. 

The high dispersive power of diamonds causes that 
fascinating display of beautiful spectral colors called 
“ first fire,” but it renders this mineral at the same time 
utterly unfit for spectacles, even if some people were 
rich enough to pay for them. 


F. —Conclusion, Containing Some Practical And 
Useful Remarks. 


1. To Redress Spectacles. In order to save time 
and trouble, we should invariably commence with the 
nose-piece in connection with that eye which is the near¬ 
est correct. We should then bend the other eye so, that 
both form a perfect plane, or that they stand in a straight 
line. Beginners do well to provide themselves with a 
small ruler about 3 inches in length, and use it as a test 
by placing it flatly on one eye, observing whether the 
other one is in the same plane. Then put it edgewise 
over the middle of one eye, from temple to nose-piece, 
and see whether the other glass is not too high or too 
low. When the middle part is corrected we examine the 
temples, and straighten them without paying any atten¬ 
tion to the position they will have in relation to the cen¬ 
tre part. If one of them extends too far to the outside, 
12 




- 108 — 


we should loosen the screw, or better, take out the glass 
altogether, and bend the joint upward, thus bringing the 
temple to a right angle with the centre. It remains now 
only to give the finishing touch to the temples. If one 
of them stands lower than the other, the lens on that 
side will be raised to the greatest disturbance of the vis¬ 
ion. To correct this, we close both temples, and see 
which one points exactly to the opposite joint; we take 
this as the model, by which we correct the other one! 
We cannot do this by bending the temple itself up or 
down, for this would undo a former correction, which 
consisted in straightening the temples “without paying 
attention to their position.’’ A little reflection soon con¬ 
vinces us that the fault is not with the temple, but with 
the joint. In order to bend the joint, we must take a 
good hold of it with some blunt cutting-pliers nearest 
the eye, leaving almost the whole length of the joint at 
our disposal, and by means of strong flat-pliers we can 
bring the joint to its proper position without any risk. 
Any bending of spectacles should be done always with 
two pliers, one in each hand. In addition to the above, 
we also need round-pliers, especially in redressing the 
nose-piece. To ascertain finally the correctness of our 
work, we must lay them edgewise upon a flat surface, as 
the show-case, and see if the ends of both temples touch 
the glass; if it does not, we have to go once more over 
the whole line of the aforesaid manipulations. 

2. To Increase the Strength of a cx lens, it is nec¬ 
essary to remove it from the eye. With a concave lens 
it is the reverse; its removal from the eye makes it 
weaker. A cc lens is strongest the nearer we approach 
it to the eye. 

3. Direct Vision is that which pertains to the ma¬ 
cula; that which belongs to the rest of the retina is 


107 — 


called indirect or peripheral vision . Indirect vision, al¬ 
though it may be very indistinct and imperfect in com¬ 
parison with central vision , is, however, not less import¬ 
ant than the latter. Without peripheric vision we would 
be in the condition of a man looking through a long, 
narrow tube which would allow of his seeing nothing but 
the object to which the axis of vision was directed. It 
would be impossible for him to see objects to one side 
without an incessant turning of his head. ( Landolt.) 

4. The Base of a Prism should be always turned in 
the opposite direction to that in which the eye is devi¬ 
ated: strabismus divergens = base in, convergens = 
base out. 

5. The Axis of Cyl. Glasses should be placed at right 
angles to the meridian we intend to correct. If the eye 
sees objects indistinctly in the vertical line, place the 
axis of the correcting cyl. lens horizontally, or alwavs 
90 ° from the faulty meridian. 

6. The Different Kinds of Spherical Lenses. 

[Page 14.] 

Double cx or cc, both curves are segments of two 
•equal spheres. 



Double cx. Double cc. 






— 108 — 

Periseopic cx or cc, the two curves are formed by 
spheres of different sizes. 



Plano cx or cc, formed by a plane and one sphere. 
The plane is either inside or outside of the sphere. 


) 

Plano cx. 


7 



Plano cc. 

























. 















































' 



























HAND-BOOK 

FOR YOUNG AND OLD 

OPTICIANS. 


A CONCISE AND COMPREHENSIVE TREATISE ON THE THEORY OF THE 
OPTICAL TRADE AND OF ITS MECHANICAL MANIPULATIONS. 


An Indispensable Companion To All Progressive Co-Laborers 

OF THE 

OPTICAL TRADE, 

CONTAINING MANY POINTS HERETOFORE UNEXPLORED AND UNEXPLAINED. 


BY 

W. BOHNE, 

OPTICIAN. 


WITH ILLUSTRATIONS. 


PUBLISHED BY THE AUTHOB, 

(With A. B GRISWOLD & CO.) 

No. 119 CANAL STREET, NEW ORLEANS, LA. 

1888 . 







diftdiliM 


Although the “Spectacle Department’ ? is one of the 
easiest of the optical business, jet it presents such diffi¬ 
culties, and involves so many scientific questions, that any 
one who will follow it, must either submit to a regular 
course of apprenticeship, or devote much of his time to 
pains-taking investigations and tedious experiments, in or¬ 
der to master his trade to the satisfaction of himself and of 
his customers. About one-tenth of all spectacle dealers 
have had the advantage of being systematically trained to 
the requirements of their trade; the others are merely 
groping in the dark. For this reason thousands of retail¬ 
ing opticians, jewelers, and dealers in optical goods daily 
incur all sorts of vexations from ill-pleased customers. 
.The author himself experienced these about thirty years 
ago. To assist my struggling companions, and spare 
them the many disappointments incident to the spectacle 
trade, I have deposited in this Hand-Book the experience 
and labors of a life-long practice as a working optician. 
In writing this instructor, I had to depend entirely upon 
my own resources. There were no previous publications 
treating of the same subjects. Nowhere, for instance, 
was any volume to be found, with directions concerning 
the delicate manipulations required to fill the various com¬ 
plicated prescriptions of oculists, a task always to be 
executed with the nice accuracy that necessarily belongs 
to spectacles and all optical instruments. 

My aim is to open the way for a new trade-literature, 
hitherto underrated and neglected. It is of the greatest 
importance to every member of the trade to be made ac¬ 
quainted with its fundamental laws and principles, and to 
be informed of its progress and development: this edu¬ 
cates the beginner, and gives new ideas to the accom¬ 
plished workman. Although the Hand-Book contains 
only 108 pages, it abounds in practical suggestions, some 
of them but little known before, such as the measuring of 
cylinders by spherical lenses (explained on page 42); or 
of prisms by means of a simple foot-rule (page 40). 
There are certainly scores of artificers able to write a 
better book, if they only would divulge their secrets; but 
an incomprehensible narrowness, the typical feature of 



all trades, forbids the competent laborer to “give away” 
his dearly bought and laboriously earned acquirements 
without compensation. A constant traffic is going on 
between the expert and inexpert in regard to information 
concerning special knacks. This is the great impediment 
in the advancement of the laboring classes generally; and 
if the different Trade-Unions would only understand their 
real mission, they would make the Education of their 
members into shillful craftsmen the principal “plank” of 
their constitutions. Watchmakers, jewelers and some 
other trades-people have already their periodicals, but we 
opticians have none. The talented editors of those pa¬ 
pers will gladly unite our interest with theirs, if able 
workmen can be induced to send them instructive manu¬ 
scripts upon the optical trade. My book advocates this 
idea throughout; and I hope it will prompt some public 
spirited co-laborers to take this in consideration, and to 
improve upon my effort. 

I claim for my book: 

1. That it provides the apprentice with a sound and 
solid foundation of the optical trade, theoretically as well 
as practically. 

2. It presents to the more advanced workman valua¬ 
ble points about important subjects, some of them hith¬ 
erto neither investigated nor correctly explained. 

3. It furnishes a clear interpretation of the Optic 
Line , and removes the great difficulties of the correct 
setting of Compound Lenses , no matter at what angle the 
axis has to be placed. 

4. It contains many interesting, and also some new in¬ 
formations concerning the eye, and should be for refer¬ 
ence in the hands of every optician as the only book, es¬ 
pecially written by a practical workman for the advance¬ 
ment of the optical trade. 

5. It gives many valuable instructions and formulas 
for saving labor, material and tools, each of them worth 
more than the price of the book. 

6. It is written in plain, intelligible English, in order 
to benefit all classes of my readers. 

The Hand-Book is now ready for delivery, and can be 
had through all leading optical wholesale houses; or it 
may be ordered directly from the author. 

IJ^g^Frice, j$2.50. 


W. BOHNE. 


CONTENTS. 


PAGE. 

CHAPTER I.—Introductory Remarks, Diopter or Diop¬ 
tric, Inch and Metric Systems. 13 

“ II.—Different Qualities of Lenses.. 21 

HI.—Pebbles—Their Merits and Defects. 28 

IV.—Definition of Spherical, Prismatical and 

Cylindrical Lenses. 35 

“ V.—Optic Line and Centre, and How to De¬ 
centre Lenses. 44 

u VI.—Fitting of Spectacle-Glasses, Their Filing 

and Drilling. 47 

VII.—Measuring of Compound Lenses and 

Their Correct Setting. 54 

“ VTTT.—Selection of Spectacles; Pupil Distance, 

Nose-Bridge,Reading and Street-Glasses 62 
“ IX.—Double-Focus Single and Split-Glasses, 

Relative to Optical Line and Centre.. 68 

“ X.—Colored or Tinted Glasses. 72 

“ XI.—The Proportion of Caloric Rays in the 

Different Kinds of Light. 78 

“ XII.—Second Sight, its Cause and Final Course. 83 

“ XIH.—History of the Invention and Introduc¬ 
tion of Spectacles. 85 

“ XIV.—Miscellaneous Appendices: 

A. —How To Relieve an Injured Eye . . 94 

B. —How Far Can We See?. 97 

C. —Why Do We Shed Tears When We 

Weep ?. 100 

D. —Different Expressions of Faces, 

Based upon the Position of the 

Eyes... 102 

E. —Refraction and Dispersion of Light 103 

F. —Conclusion, Containing Some Prac¬ 

tical and Useful Remarks. 105 


















View$ froii| tl\e Critic^. 


- «- 

New Orleans Picayune, Jan 1 y 15, 1888. 

The veteran and well-known optician, Mr. W. Bohne, 
has published a very important work, a “Hand-book for 
Opticians.” It is a candid, patient and thoroughly com¬ 
prehensive work, whose merits will be apparent to all 
connected with the optical trade. Every particular 
branch of the work of opticians is covered in this admir¬ 
able work, and for those for whom the book is intended 
it will prove indispensable. A most entertaining chapter 
concerns the invention and introduction of spectacles. 

The Metallurgist and National Jeweler, 
Chicago, February, 1888. 

The Hand-Book contains a large amount of useful and 
practical knowledge, and will prove very profitable to a 
large number of opticians. We can recommend the book 
to all dealers desiring a practical knowledge of the work¬ 
ing opticians’ business. 

New Orleans Medical and Surgical Journal, 
March 1888. 

The well-known optician of New Orleans, Mr. W. 
Bohne, has published a little book, designated as a guide 
for all young opticians, which we should think was des¬ 
tined to prove very useful. The author deals strictly 



with practical questions: the selection of good lenses, 
the advantages and disadvantages of pebbles, crown and 
flint glass, and of the various tinted glasses; the setting 
and the centering of the different sorts of lenses; the 
selection and fitting of frames, etc., closing with a very 
good little sketch of the history of the invention and in¬ 
troduction of spectacles. Great good sense has been 
shown in not casting aside many useful minutice, and we 
can imagine that the book would be a perfect godsend to 
many an ill-instructed beginner struggling with the prin¬ 
ciples of his trade. Indeed, we have no doubt that num¬ 
bers of oculists who consider themselves thoroughly in¬ 
structed, will be able to gather a good point or two from 
this book. 

The American Journal of Ophthalmology , 

St. Louis , March , 1888. 

This little Hand-Book is of value and will repay the 
reading. It is a fact, which we note with satisfaction, 
that the intelligent opticians are beginning to be more 
than mere mechanics. The more they know about the 
theory of the optical trade, the less will they be liable to 
do bad work and to practise quackery. We heartily re¬ 
commend this original effort of an optician, to opticians 
and oculists. 

The Jewelers' Circular and Horological Review , 
New York. 

The work is well illustrated, printed on good paper 
from large, clear type and substantially bound. The ne¬ 
cessity for all persons handling optical goods, having suf¬ 
ficient knowledge of the optical science to enable them to 
adjust glasses intelligently, is becoming more and more 
apparent every day. Salesmen familiar with the work 
are in demand, and command better salaries than those 


who do not. A practical book of this character will be a 
boon to many, and invaluable to all who study it. 

Johnston Eye-Echo, Detroit, March-April, 1888 . 

We have examined the Hand-Book with great pleasure. 
Several reasons make it a book of unusual interest to one 
occupied with optical questions. The object of the au¬ 
thor leads him along an unfrequented path. His address 
is to the practical optician, touching that part of his 
work which pertains to the goods and tools he handles.— 
A feature of the book which appeals to our interest with 
exceptional force is its spirit of candor and frankness. 
The writer presents the results of his own experience and 
experiments.—As to the contents, the reader’s first 
thought may be that the book is not large enough. In 
reading some chapters one may reach the end so quickly 
as to feel a sense of surprise shading off into regret. 
This is no fault but rather a virtue in this book-making 
age. The author is not writing merely to make a book, 
but prefers to impart in a direct manner a definite stock 
of information collected by him.—Some of the enter¬ 
taining hints and comments in the last chapter on collat¬ 
eral subjects have pleased us much. 

The Boston Medical and Surgical Journal , 
July, 1888 . 

The Hand-Book strives to teach the optician the theory 
and practice of his trade: how to test lenses, cut them, 
and set them in the frame; how to fit .the frames to the 
face, and to modify the frames according to the use to 
which they are to be put; how to centre and decentre 
lenses, etc. Unfortunately, the majority of opticians are 
sadly in need of just this information. The book is ju¬ 
diciously written and covers its subjects very well, and 
deserves a wide reading among the opticians, as it is the 
only work of this kind that we have seen. 


Physicians and Surgeons Investigator , Buffalo , 
August , 

A very readable and practical book. It is the only 
work of this kind in English, and to the progressive op¬ 
tical workman is simply invaluable, while the oculist will 
lind much pertaining to lens grinding and setting that is 
not found in works on the eye. It is a little inexpensive 
book, and is well worth sending for. 

Medical Journal and Examiner , Chicago , 
November, 1888 . 

This book is designed for the guidance of opticians. 
There is much in it of practical value to the working op¬ 
tician, and the advice given is generally reliable, even 
when the author goes out of his province to advise treat¬ 
ment in cases of injury to eyes from various accidents. 

Archives of Ophthalmology , New York, 
December , 1888 . 

This book with numerous diagrams, although written 
for practical opticians, will be read with benefit also by 
oculists. It gives much needed information on many 
points which are neither taught in our medical schools 
nor mentioned in our text-books. Chapters II, III, V 
and VII are the most useful for the oculist. How often 
have we to determine the kind and strength of spectacles 
worn by patients who have no formula with them. The 
problem for spherical glasses is indeed easy enough, but 
when we have to deal with sphero-cylindrical, or sphero- 
cylindro-prismatic glasses, the problem is not so simple. 
The rules for these are plainly detailed by the author and 
exemplified by a number of problems. Chapter XI des¬ 
cribes some very interesting experiments. We can fairly 
recommend the book. 



































































. 

■ 
















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